diff --git a/SConstruct b/SConstruct
index 98f47a9..720c281 100644
--- a/SConstruct
+++ b/SConstruct
@@ -272,6 +272,7 @@
                             extent_mapper_unittest.cc
                             extent_ranges_unittest.cc
                             extent_writer_unittest.cc
+                            fake_prefs.cc
                             fake_system_state.cc
                             file_writer_unittest.cc
                             filesystem_copier_action_unittest.cc
diff --git a/fake_prefs.cc b/fake_prefs.cc
new file mode 100644
index 0000000..1df4e30
--- /dev/null
+++ b/fake_prefs.cc
@@ -0,0 +1,123 @@
+// Copyright 2014 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "update_engine/fake_prefs.h"
+
+#include <gtest/gtest.h>
+
+using std::map;
+using std::string;
+
+using chromeos_update_engine::FakePrefs;
+
+namespace {
+
+void CheckNotNull(const string& key, void* ptr) {
+  EXPECT_NE(nullptr, ptr)
+      << "Called Get*() for key \"" << key << "\" with a NULL parameter.";
+}
+
+}  // namespace
+
+namespace chromeos_update_engine {
+
+// Compile-time type-dependent constants definitions.
+template<>
+FakePrefs::PrefType const FakePrefs::PrefConsts<string>::type =
+    FakePrefs::PrefType::kString;
+template<>
+string FakePrefs::PrefValue::* const FakePrefs::PrefConsts<string>::member =
+    &FakePrefs::PrefValue::as_str;
+
+template<>
+FakePrefs::PrefType const FakePrefs::PrefConsts<int64_t>::type =
+    FakePrefs::PrefType::kInt64;
+template<>
+int64_t FakePrefs::PrefValue::* const FakePrefs::PrefConsts<int64_t>::member =
+    &FakePrefs::PrefValue::as_int64;
+
+template<>
+FakePrefs::PrefType const FakePrefs::PrefConsts<bool>::type =
+    FakePrefs::PrefType::kBool;
+template<>
+bool FakePrefs::PrefValue::* const FakePrefs::PrefConsts<bool>::member =
+    &FakePrefs::PrefValue::as_bool;
+
+
+bool FakePrefs::GetString(const string& key, string* value) {
+  return GetValue(key, value);
+}
+
+bool FakePrefs::SetString(const std::string& key, const std::string& value) {
+  SetValue(key, value);
+  return true;
+}
+
+bool FakePrefs::GetInt64(const string& key, int64_t* value) {
+  return GetValue(key, value);
+}
+
+bool FakePrefs::SetInt64(const string& key, const int64_t value) {
+  SetValue(key, value);
+  return true;
+}
+
+bool FakePrefs::GetBoolean(const std::string& key, bool* value) {
+  return GetValue(key, value);
+}
+
+bool FakePrefs::SetBoolean(const string& key, const bool value) {
+  SetValue(key, value);
+  return true;
+}
+
+bool FakePrefs::Exists(const string& key) {
+  return values_.find(key) != values_.end();
+}
+
+bool FakePrefs::Delete(const string& key) {
+  if (values_.find(key) == values_.end())
+    return false;
+  values_.erase(key);
+  return true;
+}
+
+string FakePrefs::GetTypeName(PrefType type) {
+  switch (type) {
+    case PrefType::kString:
+      return "string";
+    case PrefType::kInt64:
+      return "int64_t";
+    case PrefType::kBool:
+      return "bool";
+  }
+  return "Unknown";
+}
+
+void FakePrefs::CheckKeyType(const string& key, PrefType type) const {
+  auto it = values_.find(key);
+  EXPECT_TRUE(it == values_.end() || it->second.type == type)
+      << "Key \"" << key << "\" if defined as " << GetTypeName(it->second.type)
+      << " but is accessed as a " << GetTypeName(type);
+}
+
+template<typename T>
+void FakePrefs::SetValue(const string& key, const T& value) {
+  CheckKeyType(key, PrefConsts<T>::type);
+  values_[key].type = PrefConsts<T>::type;
+  values_[key].value.*(PrefConsts<T>::member) = value;
+}
+
+template<typename T>
+bool FakePrefs::GetValue(const string& key, T* value) const {
+  CheckKeyType(key, PrefConsts<T>::type);
+  auto it = values_.find(key);
+  if (it == values_.end())
+    return false;
+  CheckNotNull(key, value);
+  *value = it->second.value.*(PrefConsts<T>::member);
+  return true;
+}
+
+}  // namespace chromeos_update_engine
diff --git a/fake_prefs.h b/fake_prefs.h
new file mode 100644
index 0000000..30c3486
--- /dev/null
+++ b/fake_prefs.h
@@ -0,0 +1,91 @@
+// Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROMEOS_PLATFORM_UPDATE_ENGINE_FAKE_PREFS_H_
+#define CHROMEOS_PLATFORM_UPDATE_ENGINE_FAKE_PREFS_H_
+
+#include <map>
+#include <string>
+
+#include <base/basictypes.h>
+
+#include "update_engine/prefs_interface.h"
+
+namespace chromeos_update_engine {
+
+// Implements a fake preference store by storing the value associated with
+// a key in a std::map, suitable for testing. It doesn't allow to set a value on
+// a key with a different type than the previously set type. This enforces the
+// type of a given key to be fixed. Also the class checks that the Get*()
+// methods aren't called on a key set with a different type.
+
+class FakePrefs : public PrefsInterface {
+ public:
+  FakePrefs() {}
+
+  // PrefsInterface methods.
+  bool GetString(const std::string& key, std::string* value) override;
+  bool SetString(const std::string& key, const std::string& value) override;
+  bool GetInt64(const std::string& key, int64_t* value) override;
+  bool SetInt64(const std::string& key, const int64_t value) override;
+  bool GetBoolean(const std::string& key, bool* value) override;
+  bool SetBoolean(const std::string& key, const bool value) override;
+
+  bool Exists(const std::string& key) override;
+  bool Delete(const std::string& key) override;
+
+ private:
+  enum class PrefType {
+    kString,
+    kInt64,
+    kBool,
+  };
+  struct PrefValue {
+    std::string as_str;
+    int64_t as_int64;
+    bool as_bool;
+  };
+
+  struct PrefTypeValue {
+    PrefType type;
+    PrefValue value;
+  };
+
+  // Class to store compile-time type dependant constants.
+  template<typename T>
+  class PrefConsts {
+   public:
+    // The PrefType associated with T.
+    static FakePrefs::PrefType const type;
+
+    // The data member pointer to PrefValue associated with T.
+    static T FakePrefs::PrefValue::* const member;
+  };
+
+  // Returns a string representation of the PrefType useful for logging.
+  static std::string GetTypeName(PrefType type);
+
+  // Checks that the |key| is either not present or has the given |type|.
+  void CheckKeyType(const std::string& key, PrefType type) const;
+
+  // Helper function to set a value of the passed |key|. It sets the type based
+  // on the template parameter T.
+  template<typename T>
+  void SetValue(const std::string& key, const T& value);
+
+  // Helper function to get a value from the map checking for invalid calls.
+  // The function fails the test if you attempt to read a value  defined as a
+  // different type. Returns whether the get succeeded.
+  template<typename T>
+  bool GetValue(const std::string& key, T* value) const;
+
+  // Container for all the key/value pairs.
+  std::map<std::string, PrefTypeValue> values_;
+
+  DISALLOW_COPY_AND_ASSIGN(FakePrefs);
+};
+
+}  // namespace chromeos_update_engine
+
+#endif  // CHROMEOS_PLATFORM_UPDATE_ENGINE_FAKE_PREFS_H_