diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 30b6e04834..9d6cad976d 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -174,6 +174,8 @@ set(TINT_LIB_SRCS
   type_manager.h
   validator.cc
   validator.h
+  validator_impl.cc
+  validator_impl.h
   # TODO(dsinclair): The writers should all be optional
   writer/spv/generator.cc
   writer/spv/generator.h
@@ -283,6 +285,7 @@ set(TINT_TEST_SRCS
   reader/wgsl/parser_impl_variable_storage_decoration_test.cc
   reader/wgsl/token_test.cc
   type_manager_test.cc
+  validator_impl_import_test.cc
 )
 
 ## Tint library
diff --git a/src/ast/module.h b/src/ast/module.h
index e0d1a7ead2..cfe26e7b77 100644
--- a/src/ast/module.h
+++ b/src/ast/module.h
@@ -43,7 +43,9 @@ class Module {
     imports_.push_back(std::move(import));
   }
   /// @returns the imports for this module
-  const std::vector<std::unique_ptr<Import>>& imports() { return imports_; }
+  const std::vector<std::unique_ptr<Import>>& imports() const {
+    return imports_;
+  }
   /// Find the import of the given name
   /// @param name The import name to search for
   /// @returns the import with the given name if found, nullptr otherwise.
diff --git a/src/validator.cc b/src/validator.cc
index 1ee1c4cac0..d190fca5ba 100644
--- a/src/validator.cc
+++ b/src/validator.cc
@@ -14,14 +14,21 @@
 
 #include "src/validator.h"
 
+#include "src/validator_impl.h"
+
 namespace tint {
 
-Validator::Validator() = default;
+Validator::Validator() : impl_(std::make_unique<tint::ValidatorImpl>()) {}
 
 Validator::~Validator() = default;
 
-bool Validator::Validate(const ast::Module&) {
-  return true;
+bool Validator::Validate(const ast::Module& module) {
+  bool ret = impl_->Validate(module);
+
+  if (impl_->has_error())
+    set_error(impl_->error());
+
+  return ret;
 }
 
 }  // namespace tint
diff --git a/src/validator.h b/src/validator.h
index 32c64b7aad..90400e1d63 100644
--- a/src/validator.h
+++ b/src/validator.h
@@ -15,12 +15,16 @@
 #ifndef SRC_VALIDATOR_H_
 #define SRC_VALIDATOR_H_
 
+#include <memory>
 #include <string>
 
 #include "src/ast/module.h"
+#include "src/validator_impl.h"
 
 namespace tint {
 
+class ValidatorImpl;
+
 /// Determines if the module is complete and valid
 class Validator {
  public:
@@ -35,8 +39,14 @@ class Validator {
 
   /// @returns error messages from the validator
   const std::string& error() { return error_; }
+  /// @returns true if an error was encountered
+  bool has_error() const { return error_.size() > 0; }
+  /// Sets the error string
+  /// @param msg the error message
+  void set_error(const std::string& msg) { error_ = msg; }
 
  private:
+  std::unique_ptr<ValidatorImpl> impl_;
   std::string error_;
 };
 
diff --git a/src/validator_impl.cc b/src/validator_impl.cc
new file mode 100644
index 0000000000..57fadba631
--- /dev/null
+++ b/src/validator_impl.cc
@@ -0,0 +1,44 @@
+// Copyright 2020 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "src/validator_impl.h"
+
+namespace tint {
+
+ValidatorImpl::ValidatorImpl() = default;
+
+ValidatorImpl::~ValidatorImpl() = default;
+
+void ValidatorImpl::set_error(const Source& src, const std::string& msg) {
+  error_ =
+      std::to_string(src.line) + ":" + std::to_string(src.column) + ": " + msg;
+}
+
+bool ValidatorImpl::Validate(const ast::Module& module) {
+  if (!CheckImports(module))
+    return false;
+  return true;
+}
+
+bool ValidatorImpl::CheckImports(const ast::Module& module) {
+  for (const auto& import : module.imports()) {
+    if (import->path() != "GLSL.std.450") {
+      set_error(import->source(), "v-0001: unknown import: " + import->path());
+      return false;
+    }
+  }
+  return true;
+}
+
+}  // namespace tint
diff --git a/src/validator_impl.h b/src/validator_impl.h
new file mode 100644
index 0000000000..414bf48ff4
--- /dev/null
+++ b/src/validator_impl.h
@@ -0,0 +1,58 @@
+// Copyright 2020 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef SRC_VALIDATOR_IMPL_H_
+#define SRC_VALIDATOR_IMPL_H_
+
+#include <string>
+
+#include "src/ast/module.h"
+
+namespace tint {
+
+/// Determines if the module is complete and valid
+class ValidatorImpl {
+ public:
+  /// Constructor
+  ValidatorImpl();
+  ~ValidatorImpl();
+
+  /// Runs the validator
+  /// @param module the module to validate
+  /// @returns true if the validation was successful
+  bool Validate(const ast::Module& module);
+
+  /// @returns error messages from the validator
+  const std::string& error() { return error_; }
+
+  /// @returns true if an error was encountered
+  bool has_error() const { return error_.size() > 0; }
+
+  /// Sets the error string
+  /// @param src the source causing the error
+  /// @param msg the error message
+  void set_error(const Source& src, const std::string& msg);
+
+  /// Validates v-0001: Only allowed import is "GLSL.std.450"
+  /// @param module the modele to check imports
+  /// @returns ture if input complies with v-0001 rule
+  bool CheckImports(const ast::Module& module);
+
+ private:
+  std::string error_;
+};
+
+}  // namespace tint
+
+#endif  // SRC_VALIDATOR_IMPL_H_
diff --git a/src/validator_impl_import_test.cc b/src/validator_impl_import_test.cc
new file mode 100644
index 0000000000..5b17e064ab
--- /dev/null
+++ b/src/validator_impl_import_test.cc
@@ -0,0 +1,56 @@
+// Copyright 2020 The Tint Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <iostream>
+#include "gtest/gtest.h"
+#include "src/reader/wgsl/parser.h"
+#include "src/validator_impl.h"
+
+namespace tint {
+
+using ValidatorImplTest = testing::Test;
+
+ast::Module build_module(std::string data) {
+  auto reader = std::make_unique<tint::reader::wgsl::Parser>(
+      std::string(data.begin(), data.end()));
+  assert(reader->Parse());
+  return reader->module();
+}
+
+TEST_F(ValidatorImplTest, Import) {
+  std::string input = "import \"GLSL.std.450\" as glsl;";
+  auto module = build_module(input);
+  tint::ValidatorImpl v;
+  EXPECT_TRUE(v.CheckImports(module));
+}
+
+TEST_F(ValidatorImplTest, Import_Fail_NotGLSL) {
+  std::string input = "import \"not.GLSL\" as glsl;";
+  auto module = build_module(input);
+  tint::ValidatorImpl v;
+  EXPECT_FALSE(v.CheckImports(module));
+  ASSERT_TRUE(v.has_error());
+  EXPECT_EQ(v.error(), "1:1: v-0001: unknown import: not.GLSL");
+}
+
+TEST_F(ValidatorImplTest, Import_Fail_Typo) {
+  std::string input = "import \"GLSL.std.4501\" as glsl;";
+  auto module = build_module(input);
+  tint::ValidatorImpl v;
+  EXPECT_FALSE(v.CheckImports(module));
+  ASSERT_TRUE(v.has_error());
+  EXPECT_EQ(v.error(), "1:1: v-0001: unknown import: GLSL.std.4501");
+}
+
+}  // namespace tint