diff --git a/clamp-0.6.3.gemspec b/clamp-0.6.3.gemspec
index 8f51cc3..acab485 100644
--- clamp-0.6.3.gemspec
+++ clamp-0.6.3.gemspec
@@ -43,6 +43,7 @@
 - lib/clamp/command.rb
 - lib/clamp/errors.rb
 - lib/clamp/help.rb
+- lib/clamp/messages.rb
 - lib/clamp/option/declaration.rb
 - lib/clamp/option/definition.rb
 - lib/clamp/option/parsing.rb

diff --git a/lib/clamp/attribute/instance.rb b/lib/clamp/attribute/instance.rb
index 8f51cc3..acab485 100644
--- lib/clamp/attribute/instance.rb
+++ lib/clamp/attribute/instance.rb
@@ -72,7 +72,7 @@ def default_from_environment
         begin
           take(value)
         rescue ArgumentError => e
-          command.send(:signal_usage_error, "$#{attribute.environment_variable}: #{e.message}")
+          command.send(:signal_usage_error, Clamp.message(:env_argument_error, :env => attribute.environment_variable, :message => e.message))
         end
       end
 
diff --git a/lib/clamp/command.rb b/lib/clamp/command.rb
index c6c50f4..52187ab 100644
--- lib/clamp/command.rb
+++ lib/clamp/command.rb
@@ -1,3 +1,4 @@
+require 'clamp/messages'
 require 'clamp/errors'
 require 'clamp/help'
 require 'clamp/option/declaration'
@@ -91,7 +92,7 @@ def help
 
     def handle_remaining_arguments
       unless remaining_arguments.empty?
-        signal_usage_error "too many arguments"
+        signal_usage_error Clamp.message(:too_many_arguments)
       end
     end
 
diff --git a/lib/clamp/help.rb b/lib/clamp/help.rb
index e09731a..1e57530 100644
--- lib/clamp/help.rb
+++ lib/clamp/help.rb
@@ -61,7 +61,7 @@ def string
       end
 
       def add_usage(invocation_path, usage_descriptions)
-        puts "Usage:"
+        puts usage_heading
         usage_descriptions.each do |usage|
           puts "    #{invocation_path} #{usage}".rstrip
         end
@@ -87,6 +87,12 @@ def add_list(heading, items)
         end
       end
 
+      protected
+
+      def usage_heading
+        "Usage:"
+      end
+
       private
 
       def puts(*args)
diff --git a/lib/clamp/messages.rb b/lib/clamp/messages.rb
new file mode 100644
index 0000000..61cce94
--- /dev/null
+++ lib/clamp/messages.rb
@@ -0,0 +1,39 @@
+module Clamp
+
+  def self.messages=(messages)
+    @user_defined_messages = messages
+  end
+
+  def self.message(key, options={})
+    @user_defined_messages ||= {}
+    msg = @user_defined_messages[key] || messages[key]
+    format_string(msg, options)
+  end
+
+  def self.messages
+    {
+      :too_many_arguments => "too many arguments",
+      :option_required => "option '%<option>s' is required",
+      :option_or_env_required => "option '%<option>s' (or env %<env>s) is required",
+      :option_argument_error => "option '%<switch>s': %<message>s",
+      :parameter_argument_error => "parameter '%<param>s': %<message>s",
+      :env_argument_error => "$%<env>s: %<message>s",
+      :unrecognised_option => "Unrecognised option '%<switch>s'",
+      :no_such_subcommand => "No such sub-command '%<name>s'",
+      :no_value_provided => "no value provided"
+    }
+  end
+
+  private
+
+  # string formatting for ruby 1.8
+  def self.format_string(string, params)
+    array_params = string.scan(/%[<{]([^>}]*)[>}]/).collect do |name|
+      name = name[0]
+      params[name.to_s] || params[name.to_sym]
+    end
+
+    string.gsub(/%[<]([^>]*)[>]/, '%').gsub(/%[{]([^}]*)[}]/, '%s') % array_params
+  end
+
+end
diff --git a/lib/clamp/option/parsing.rb b/lib/clamp/option/parsing.rb
index 9f057f1..bb7b41c 100644
--- lib/clamp/option/parsing.rb
+++ lib/clamp/option/parsing.rb
@@ -31,7 +31,7 @@ def parse_options
           begin
             option.of(self).take(value)
           rescue ArgumentError => e
-            signal_usage_error "option '#{switch}': #{e.message}"
+            signal_usage_error Clamp.message(:option_argument_error, :switch => switch, :message => e.message)
           end
 
         end
@@ -45,11 +45,11 @@ def parse_options
         self.class.recognised_options.each do |option|
           # If this option is required and the value is nil, there's an error.
           if option.required? and send(option.attribute_name).nil?
-            message = "option '#{option.switches.first}'"
             if option.environment_variable
-              message += " (or env #{option.environment_variable})"
+              message = Clamp.message(:option_or_env_required, :option => option.switches.first, :env => option.environment_variable)
+            else
+              message = Clamp.message(:option_required, :option => option.switches.first)
             end
-            message += " is required"
             signal_usage_error message
           end
         end
@@ -59,7 +59,7 @@ def parse_options
 
       def find_option(switch)
         self.class.find_option(switch) ||
-        signal_usage_error("Unrecognised option '#{switch}'")
+        signal_usage_error(Clamp.message(:unrecognised_option, :switch => switch))
       end
 
     end
diff --git a/lib/clamp/parameter/definition.rb b/lib/clamp/parameter/definition.rb
index 6412546..a276dea 100644
--- lib/clamp/parameter/definition.rb
+++ lib/clamp/parameter/definition.rb
@@ -22,7 +22,7 @@ def help_lhs
       end
 
       def consume(arguments)
-        raise ArgumentError, "no value provided" if required? && arguments.empty?
+        raise ArgumentError, Clamp.message(:no_value_provided) if required? && arguments.empty?
         arguments.shift(multivalued? ? arguments.length : 1)
       end
 
diff --git a/lib/clamp/parameter/parsing.rb b/lib/clamp/parameter/parsing.rb
index 8a70719..95aa422 100644
--- lib/clamp/parameter/parsing.rb
+++ lib/clamp/parameter/parsing.rb
@@ -13,7 +13,7 @@ def parse_parameters
               parameter.of(self).take(value)
             end
           rescue ArgumentError => e
-            signal_usage_error "parameter '#{parameter.name}': #{e.message}"
+            signal_usage_error Clamp.message(:parameter_argument_error, :param => parameter.name, :message => e.message)
           end
         end
 
diff --git a/lib/clamp/subcommand/execution.rb b/lib/clamp/subcommand/execution.rb
index d15c56b..c341df6 100644
--- lib/clamp/subcommand/execution.rb
+++ lib/clamp/subcommand/execution.rb
@@ -25,7 +25,7 @@ def instatiate_subcommand(name)
       end
 
       def find_subcommand_class(name)
-        subcommand_def = self.class.find_subcommand(name) || signal_usage_error("No such sub-command '#{name}'")
+        subcommand_def = self.class.find_subcommand(name) || signal_usage_error(Clamp.message(:no_such_subcommand, :name => name))
         subcommand_def.subcommand_class
       end