diff --git a/CHANGELOG.md b/CHANGELOG.md
index f27b54e7e..00a64107a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,7 +5,7 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## Unreleased
+## 0.8.0 - Unreleased
 
 ### Additions
 
@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   - `setup` will generate a pyproject.toml with no Poetry information, and instead create a `setup.py` with the
     project info.
   - `none` will not create a project folder at all, only the inner package folder (which won't be inner anymore)
+- Attempt to detect and alert users if they are using an unsupported version of OpenAPI (#281).
 
 ### Changes
 
diff --git a/openapi_python_client/parser/openapi.py b/openapi_python_client/parser/openapi.py
index 3053ee305..d4ab60244 100644
--- a/openapi_python_client/parser/openapi.py
+++ b/openapi_python_client/parser/openapi.py
@@ -260,12 +260,22 @@ class GeneratorData:
     enums: Dict[str, EnumProperty]
 
     @staticmethod
-    def from_dict(d: Dict[str, Dict[str, Any]]) -> Union["GeneratorData", GeneratorError]:
+    def from_dict(d: Dict[str, Any]) -> Union["GeneratorData", GeneratorError]:
         """ Create an OpenAPI from dict """
         try:
             openapi = oai.OpenAPI.parse_obj(d)
         except ValidationError as e:
-            return GeneratorError(header="Failed to parse OpenAPI document", detail=str(e))
+            detail = str(e)
+            if "swagger" in d:
+                detail = (
+                    "You may be trying to use a Swagger document; this is not supported by this project.\n\n" + detail
+                )
+            return GeneratorError(header="Failed to parse OpenAPI document", detail=detail)
+        if openapi.openapi.major != 3:
+            return GeneratorError(
+                header="openapi-python-client only supports OpenAPI 3.x",
+                detail=f"The version of the provided document was {openapi.openapi}",
+            )
         if openapi.components is None or openapi.components.schemas is None:
             schemas = Schemas()
         else:
diff --git a/openapi_python_client/schema/__init__.py b/openapi_python_client/schema/__init__.py
index 9edb7d3d9..39ba1f456 100644
--- a/openapi_python_client/schema/__init__.py
+++ b/openapi_python_client/schema/__init__.py
@@ -1,69 +1,50 @@
-"""
-OpenAPI v3.0.3 schema types, created according to the specification:
-https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.3.md
-
-The type orders are according to the contents of the specification:
-https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.3.md#table-of-contents
-"""
-
 __all__ = [
-    "Components",
-    "Contact",
-    "Discriminator",
-    "Encoding",
-    "Example",
-    "ExternalDocumentation",
-    "Header",
-    "Info",
-    "License",
-    "Link",
     "MediaType",
-    "OAuthFlow",
-    "OAuthFlows",
     "OpenAPI",
     "Operation",
     "Parameter",
     "PathItem",
-    "Paths",
     "Reference",
     "RequestBody",
     "Response",
     "Responses",
     "Schema",
-    "SecurityRequirement",
-    "SecurityScheme",
-    "Server",
-    "ServerVariable",
-    "Tag",
-    "XML",
 ]
 
-from .components import Components
-from .contact import Contact
-from .discriminator import Discriminator
-from .encoding import Encoding
-from .example import Example
-from .external_documentation import ExternalDocumentation
-from .header import Header
-from .info import Info
-from .license import License
-from .link import Link
-from .media_type import MediaType
-from .oauth_flow import OAuthFlow
-from .oauth_flows import OAuthFlows
-from .open_api import OpenAPI
-from .operation import Operation
-from .parameter import Parameter
-from .path_item import PathItem
-from .paths import Paths
-from .reference import Reference
-from .request_body import RequestBody
-from .response import Response
-from .responses import Responses
-from .schema import Schema
-from .security_requirement import SecurityRequirement
-from .security_scheme import SecurityScheme
-from .server import Server
-from .server_variable import ServerVariable
-from .tag import Tag
-from .xml import XML
+
+import re
+from typing import Callable, Iterator
+
+from .openapi_schema_pydantic import MediaType
+from .openapi_schema_pydantic import OpenAPI as _OpenAPI
+from .openapi_schema_pydantic import Operation, Parameter, PathItem, Reference, RequestBody, Response, Responses, Schema
+
+regex = re.compile(r"(0|[1-9]\d*)\.(0|[1-9]\d*)\.(0|[1-9]\d*)")
+
+
+class SemVer:
+    def __init__(self, str_value: str) -> None:
+        self.str_value = str_value
+        if not isinstance(str_value, str):
+            raise TypeError("string required")
+        m = regex.fullmatch(str_value)
+        if not m:
+            raise ValueError("invalid semantic versioning format")
+        self.major = int(m.group(1))
+        self.minor = int(m.group(2))
+        self.patch = int(m.group(3))
+
+    @classmethod
+    def __get_validators__(cls) -> Iterator[Callable[[str], "SemVer"]]:
+        yield cls.validate
+
+    @classmethod
+    def validate(cls, v: str) -> "SemVer":
+        return cls(v)
+
+    def __str__(self) -> str:
+        return self.str_value
+
+
+class OpenAPI(_OpenAPI):
+    openapi: SemVer
diff --git a/openapi_python_client/schema/LICENSE b/openapi_python_client/schema/openapi_schema_pydantic/LICENSE
similarity index 100%
rename from openapi_python_client/schema/LICENSE
rename to openapi_python_client/schema/openapi_schema_pydantic/LICENSE
diff --git a/openapi_python_client/schema/README.md b/openapi_python_client/schema/openapi_schema_pydantic/README.md
similarity index 86%
rename from openapi_python_client/schema/README.md
rename to openapi_python_client/schema/openapi_schema_pydantic/README.md
index d5fae02d5..0e4d40146 100644
--- a/openapi_python_client/schema/README.md
+++ b/openapi_python_client/schema/openapi_schema_pydantic/README.md
@@ -1,4 +1,4 @@
-# OpenAPI v3.0.3 schema classes
+Everything in this directory (including the rest of this file after this paragraph) is a vendored copy of [openapi-schem-pydantic](https://github.com/kuimono/openapi-schema-pydantic) and is licensed under the LICENSE file in this directory.
 
 ## Alias
 
diff --git a/openapi_python_client/schema/openapi_schema_pydantic/__init__.py b/openapi_python_client/schema/openapi_schema_pydantic/__init__.py
new file mode 100644
index 000000000..9edb7d3d9
--- /dev/null
+++ b/openapi_python_client/schema/openapi_schema_pydantic/__init__.py
@@ -0,0 +1,69 @@
+"""
+OpenAPI v3.0.3 schema types, created according to the specification:
+https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.3.md
+
+The type orders are according to the contents of the specification:
+https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.3.md#table-of-contents
+"""
+
+__all__ = [
+    "Components",
+    "Contact",
+    "Discriminator",
+    "Encoding",
+    "Example",
+    "ExternalDocumentation",
+    "Header",
+    "Info",
+    "License",
+    "Link",
+    "MediaType",
+    "OAuthFlow",
+    "OAuthFlows",
+    "OpenAPI",
+    "Operation",
+    "Parameter",
+    "PathItem",
+    "Paths",
+    "Reference",
+    "RequestBody",
+    "Response",
+    "Responses",
+    "Schema",
+    "SecurityRequirement",
+    "SecurityScheme",
+    "Server",
+    "ServerVariable",
+    "Tag",
+    "XML",
+]
+
+from .components import Components
+from .contact import Contact
+from .discriminator import Discriminator
+from .encoding import Encoding
+from .example import Example
+from .external_documentation import ExternalDocumentation
+from .header import Header
+from .info import Info
+from .license import License
+from .link import Link
+from .media_type import MediaType
+from .oauth_flow import OAuthFlow
+from .oauth_flows import OAuthFlows
+from .open_api import OpenAPI
+from .operation import Operation
+from .parameter import Parameter
+from .path_item import PathItem
+from .paths import Paths
+from .reference import Reference
+from .request_body import RequestBody
+from .response import Response
+from .responses import Responses
+from .schema import Schema
+from .security_requirement import SecurityRequirement
+from .security_scheme import SecurityScheme
+from .server import Server
+from .server_variable import ServerVariable
+from .tag import Tag
+from .xml import XML
diff --git a/openapi_python_client/schema/components.py b/openapi_python_client/schema/openapi_schema_pydantic/components.py
similarity index 100%
rename from openapi_python_client/schema/components.py
rename to openapi_python_client/schema/openapi_schema_pydantic/components.py
diff --git a/openapi_python_client/schema/contact.py b/openapi_python_client/schema/openapi_schema_pydantic/contact.py
similarity index 100%
rename from openapi_python_client/schema/contact.py
rename to openapi_python_client/schema/openapi_schema_pydantic/contact.py
diff --git a/openapi_python_client/schema/discriminator.py b/openapi_python_client/schema/openapi_schema_pydantic/discriminator.py
similarity index 100%
rename from openapi_python_client/schema/discriminator.py
rename to openapi_python_client/schema/openapi_schema_pydantic/discriminator.py
diff --git a/openapi_python_client/schema/encoding.py b/openapi_python_client/schema/openapi_schema_pydantic/encoding.py
similarity index 100%
rename from openapi_python_client/schema/encoding.py
rename to openapi_python_client/schema/openapi_schema_pydantic/encoding.py
diff --git a/openapi_python_client/schema/example.py b/openapi_python_client/schema/openapi_schema_pydantic/example.py
similarity index 100%
rename from openapi_python_client/schema/example.py
rename to openapi_python_client/schema/openapi_schema_pydantic/example.py
diff --git a/openapi_python_client/schema/external_documentation.py b/openapi_python_client/schema/openapi_schema_pydantic/external_documentation.py
similarity index 100%
rename from openapi_python_client/schema/external_documentation.py
rename to openapi_python_client/schema/openapi_schema_pydantic/external_documentation.py
diff --git a/openapi_python_client/schema/header.py b/openapi_python_client/schema/openapi_schema_pydantic/header.py
similarity index 100%
rename from openapi_python_client/schema/header.py
rename to openapi_python_client/schema/openapi_schema_pydantic/header.py
diff --git a/openapi_python_client/schema/info.py b/openapi_python_client/schema/openapi_schema_pydantic/info.py
similarity index 100%
rename from openapi_python_client/schema/info.py
rename to openapi_python_client/schema/openapi_schema_pydantic/info.py
diff --git a/openapi_python_client/schema/license.py b/openapi_python_client/schema/openapi_schema_pydantic/license.py
similarity index 100%
rename from openapi_python_client/schema/license.py
rename to openapi_python_client/schema/openapi_schema_pydantic/license.py
diff --git a/openapi_python_client/schema/link.py b/openapi_python_client/schema/openapi_schema_pydantic/link.py
similarity index 100%
rename from openapi_python_client/schema/link.py
rename to openapi_python_client/schema/openapi_schema_pydantic/link.py
diff --git a/openapi_python_client/schema/media_type.py b/openapi_python_client/schema/openapi_schema_pydantic/media_type.py
similarity index 100%
rename from openapi_python_client/schema/media_type.py
rename to openapi_python_client/schema/openapi_schema_pydantic/media_type.py
diff --git a/openapi_python_client/schema/oauth_flow.py b/openapi_python_client/schema/openapi_schema_pydantic/oauth_flow.py
similarity index 100%
rename from openapi_python_client/schema/oauth_flow.py
rename to openapi_python_client/schema/openapi_schema_pydantic/oauth_flow.py
diff --git a/openapi_python_client/schema/oauth_flows.py b/openapi_python_client/schema/openapi_schema_pydantic/oauth_flows.py
similarity index 100%
rename from openapi_python_client/schema/oauth_flows.py
rename to openapi_python_client/schema/openapi_schema_pydantic/oauth_flows.py
diff --git a/openapi_python_client/schema/open_api.py b/openapi_python_client/schema/openapi_schema_pydantic/open_api.py
similarity index 83%
rename from openapi_python_client/schema/open_api.py
rename to openapi_python_client/schema/openapi_schema_pydantic/open_api.py
index abec6cc01..dd480a8f5 100644
--- a/openapi_python_client/schema/open_api.py
+++ b/openapi_python_client/schema/openapi_schema_pydantic/open_api.py
@@ -14,14 +14,6 @@
 class OpenAPI(BaseModel):
     """This is the root document object of the OpenAPI document."""
 
-    openapi: str = "3.0.3"
-    """
-    **REQUIRED**. This string MUST be the [semantic version number](https://semver.org/spec/v2.0.0.html)
-    of the [OpenAPI Specification version](#versions) that the OpenAPI document uses.
-    The `openapi` field SHOULD be used by tooling specifications and clients to interpret the OpenAPI document.
-    This is *not* related to the API [`info.version`](#infoVersion) string.
-    """
-
     info: Info
     """
     **REQUIRED**. Provides metadata about the API. The metadata MAY be used by tooling as required.
diff --git a/openapi_python_client/schema/operation.py b/openapi_python_client/schema/openapi_schema_pydantic/operation.py
similarity index 100%
rename from openapi_python_client/schema/operation.py
rename to openapi_python_client/schema/openapi_schema_pydantic/operation.py
diff --git a/openapi_python_client/schema/parameter.py b/openapi_python_client/schema/openapi_schema_pydantic/parameter.py
similarity index 100%
rename from openapi_python_client/schema/parameter.py
rename to openapi_python_client/schema/openapi_schema_pydantic/parameter.py
diff --git a/openapi_python_client/schema/path_item.py b/openapi_python_client/schema/openapi_schema_pydantic/path_item.py
similarity index 100%
rename from openapi_python_client/schema/path_item.py
rename to openapi_python_client/schema/openapi_schema_pydantic/path_item.py
diff --git a/openapi_python_client/schema/paths.py b/openapi_python_client/schema/openapi_schema_pydantic/paths.py
similarity index 100%
rename from openapi_python_client/schema/paths.py
rename to openapi_python_client/schema/openapi_schema_pydantic/paths.py
diff --git a/openapi_python_client/schema/reference.py b/openapi_python_client/schema/openapi_schema_pydantic/reference.py
similarity index 100%
rename from openapi_python_client/schema/reference.py
rename to openapi_python_client/schema/openapi_schema_pydantic/reference.py
diff --git a/openapi_python_client/schema/request_body.py b/openapi_python_client/schema/openapi_schema_pydantic/request_body.py
similarity index 100%
rename from openapi_python_client/schema/request_body.py
rename to openapi_python_client/schema/openapi_schema_pydantic/request_body.py
diff --git a/openapi_python_client/schema/response.py b/openapi_python_client/schema/openapi_schema_pydantic/response.py
similarity index 100%
rename from openapi_python_client/schema/response.py
rename to openapi_python_client/schema/openapi_schema_pydantic/response.py
diff --git a/openapi_python_client/schema/responses.py b/openapi_python_client/schema/openapi_schema_pydantic/responses.py
similarity index 100%
rename from openapi_python_client/schema/responses.py
rename to openapi_python_client/schema/openapi_schema_pydantic/responses.py
diff --git a/openapi_python_client/schema/schema.py b/openapi_python_client/schema/openapi_schema_pydantic/schema.py
similarity index 100%
rename from openapi_python_client/schema/schema.py
rename to openapi_python_client/schema/openapi_schema_pydantic/schema.py
diff --git a/openapi_python_client/schema/security_requirement.py b/openapi_python_client/schema/openapi_schema_pydantic/security_requirement.py
similarity index 100%
rename from openapi_python_client/schema/security_requirement.py
rename to openapi_python_client/schema/openapi_schema_pydantic/security_requirement.py
diff --git a/openapi_python_client/schema/security_scheme.py b/openapi_python_client/schema/openapi_schema_pydantic/security_scheme.py
similarity index 100%
rename from openapi_python_client/schema/security_scheme.py
rename to openapi_python_client/schema/openapi_schema_pydantic/security_scheme.py
diff --git a/openapi_python_client/schema/server.py b/openapi_python_client/schema/openapi_schema_pydantic/server.py
similarity index 100%
rename from openapi_python_client/schema/server.py
rename to openapi_python_client/schema/openapi_schema_pydantic/server.py
diff --git a/openapi_python_client/schema/server_variable.py b/openapi_python_client/schema/openapi_schema_pydantic/server_variable.py
similarity index 100%
rename from openapi_python_client/schema/server_variable.py
rename to openapi_python_client/schema/openapi_schema_pydantic/server_variable.py
diff --git a/openapi_python_client/schema/tag.py b/openapi_python_client/schema/openapi_schema_pydantic/tag.py
similarity index 100%
rename from openapi_python_client/schema/tag.py
rename to openapi_python_client/schema/openapi_schema_pydantic/tag.py
diff --git a/openapi_python_client/schema/xml.py b/openapi_python_client/schema/openapi_schema_pydantic/xml.py
similarity index 100%
rename from openapi_python_client/schema/xml.py
rename to openapi_python_client/schema/openapi_schema_pydantic/xml.py
diff --git a/tests/test_parser/test_openapi.py b/tests/test_parser/test_openapi.py
index e3caafa9e..5c2caa3cc 100644
--- a/tests/test_parser/test_openapi.py
+++ b/tests/test_parser/test_openapi.py
@@ -14,6 +14,7 @@ def test_from_dict(self, mocker):
         EndpointCollection.from_data.return_value = (endpoints_collections_by_tag, schemas)
         OpenAPI = mocker.patch(f"{MODULE_NAME}.oai.OpenAPI")
         openapi = OpenAPI.parse_obj.return_value
+        openapi.openapi = mocker.MagicMock(major=3)
 
         in_dict = mocker.MagicMock()
 
@@ -54,16 +55,63 @@ def test_from_dict_invalid_schema(self, mocker):
         assert generator_data == GeneratorError(
             header="Failed to parse OpenAPI document",
             detail=(
-                "2 validation errors for OpenAPI\n"
+                "3 validation errors for OpenAPI\n"
                 "info\n"
                 "  field required (type=value_error.missing)\n"
                 "paths\n"
+                "  field required (type=value_error.missing)\n"
+                "openapi\n"
                 "  field required (type=value_error.missing)"
             ),
         )
         Schemas.build.assert_not_called()
         Schemas.assert_not_called()
 
+    def test_swagger_document_invalid_schema(self, mocker):
+        Schemas = mocker.patch(f"{MODULE_NAME}.Schemas")
+
+        in_dict = {"swagger": "2.0"}
+
+        from openapi_python_client.parser.openapi import GeneratorData
+
+        generator_data = GeneratorData.from_dict(in_dict)
+
+        assert generator_data == GeneratorError(
+            header="Failed to parse OpenAPI document",
+            detail=(
+                "You may be trying to use a Swagger document; this is not supported by this project.\n\n"
+                "3 validation errors for OpenAPI\n"
+                "info\n"
+                "  field required (type=value_error.missing)\n"
+                "paths\n"
+                "  field required (type=value_error.missing)\n"
+                "openapi\n"
+                "  field required (type=value_error.missing)"
+            ),
+        )
+        Schemas.build.assert_not_called()
+        Schemas.assert_not_called()
+
+    def test_from_dict_invalid_version(self, mocker):
+        Schemas = mocker.patch(f"{MODULE_NAME}.Schemas")
+
+        OpenAPI = mocker.patch(f"{MODULE_NAME}.oai.OpenAPI")
+        openapi = OpenAPI.parse_obj.return_value
+        openapi.openapi = oai.SemVer("2.1.3")
+
+        in_dict = mocker.MagicMock()
+
+        from openapi_python_client.parser.openapi import GeneratorData
+
+        generator_data = GeneratorData.from_dict(in_dict)
+
+        assert generator_data == GeneratorError(
+            header="openapi-python-client only supports OpenAPI 3.x",
+            detail="The version of the provided document was 2.1.3",
+        )
+        Schemas.build.assert_not_called()
+        Schemas.assert_not_called()
+
 
 class TestEndpoint:
     def test_parse_request_form_body(self, mocker):
diff --git a/tests/test_schema/test_open_api.py b/tests/test_schema/test_open_api.py
new file mode 100644
index 000000000..e332e4fca
--- /dev/null
+++ b/tests/test_schema/test_open_api.py
@@ -0,0 +1,16 @@
+import pytest
+from pydantic import ValidationError
+
+from openapi_python_client.schema import OpenAPI
+
+
+@pytest.mark.parametrize(
+    "version, valid", [("abc", False), ("1", False), ("2.0", False), ("3.0.0", True), ("3.1.0-b.3", False), (1, False)]
+)
+def test_validate_version(version, valid):
+    data = {"openapi": version, "info": {"title": "test", "version": ""}, "paths": {}}
+    if valid:
+        OpenAPI.parse_obj(data)
+    else:
+        with pytest.raises(ValidationError):
+            OpenAPI.parse_obj(data)