diff --git a/i18n.gemspec b/i18n.gemspec
index 5f20c15d..e70edbb2 100644
--- a/i18n.gemspec
+++ b/i18n.gemspec
@@ -19,4 +19,6 @@ Gem::Specification.new do |s|
   s.rubyforge_project = '[none]'
   s.required_rubygems_version = '>= 1.3.5'
   s.required_ruby_version = '>= 1.9.3'
+
+  s.add_dependency 'concurrent-ruby', '~> 1.0'
 end
diff --git a/lib/i18n.rb b/lib/i18n.rb
index 8c1c9de6..3946a641 100644
--- a/lib/i18n.rb
+++ b/lib/i18n.rb
@@ -1,3 +1,5 @@
+require 'concurrent/map'
+
 require 'i18n/version'
 require 'i18n/exceptions'
 require 'i18n/interpolate/ruby'
@@ -12,6 +14,11 @@ module I18n
   RESERVED_KEYS = [:scope, :default, :separator, :resolve, :object, :fallback, :fallback_in_progress, :format, :cascade, :throw, :raise, :deep_interpolation]
   RESERVED_KEYS_PATTERN = /%\{(#{RESERVED_KEYS.join("|")})\}/
 
+
+  def self.new_double_nested_cache # :nodoc:
+    Concurrent::Map.new { |h,k| h[k] = Concurrent::Map.new }
+  end
+
   module Base
     # Gets I18n configuration object.
     def config
@@ -321,8 +328,10 @@ def handle_exception(handling, exception, locale, key, options)
       end
     end
 
+    @@normalized_key_cache = I18n.new_double_nested_cache
+
     def normalize_key(key, separator)
-      normalized_key_cache[separator][key] ||=
+      @@normalized_key_cache[separator][key] ||=
         case key
         when Array
           key.map { |k| normalize_key(k, separator) }.flatten
@@ -333,10 +342,6 @@ def normalize_key(key, separator)
           keys
         end
     end
-
-    def normalized_key_cache
-      @normalized_key_cache ||= Hash.new { |h,k| h[k] = {} }
-    end
   end
 
   extend Base
diff --git a/lib/i18n/backend/flatten.rb b/lib/i18n/backend/flatten.rb
index c23f7c13..995c3462 100644
--- a/lib/i18n/backend/flatten.rb
+++ b/lib/i18n/backend/flatten.rb
@@ -43,7 +43,7 @@ def normalize_flat_keys(locale, key, scope, separator)
 
       # Store flattened links.
       def links
-        @links ||= Hash.new { |h,k| h[k] = {} }
+        @links ||= I18n.new_double_nested_cache
       end
 
       # Flatten keys for nested Hashes by chaining up keys:
@@ -99,7 +99,7 @@ def resolve_link(locale, key)
         end
 
         def find_link(locale, key) #:nodoc:
-          links[locale].each do |from, to|
+          links[locale].each_pair do |from, to|
             return [from, to] if key[0, from.length] == from
           end && nil
         end
@@ -110,4 +110,4 @@ def escape_default_separator(key) #:nodoc:
 
     end
   end
-end
\ No newline at end of file
+end
diff --git a/lib/i18n/backend/memoize.rb b/lib/i18n/backend/memoize.rb
index ae9801fc..a11bdec1 100644
--- a/lib/i18n/backend/memoize.rb
+++ b/lib/i18n/backend/memoize.rb
@@ -34,7 +34,7 @@ def lookup(locale, key, scope = nil, options = {})
         end
 
         def memoized_lookup
-          @memoized_lookup ||= Hash.new { |h, k| h[k] = {} }
+          @memoized_lookup ||= I18n.new_double_nested_cache
         end
 
         def reset_memoizations!(locale=nil)