diff --git a/lib/execjs.rb b/lib/execjs.rb
index 1a47f4d..4b5de07 100644
--- a/lib/execjs.rb
+++ b/lib/execjs.rb
@@ -1,53 +1,5 @@
-require "rbconfig"
+require "execjs/runtimes"
 
 module ExecJS
-  VERSION = "1.0.0"
-
-  class Error           < ::StandardError; end
-  class RuntimeError              < Error; end
-  class ProgramError              < Error; end
-  class RuntimeUnavailable < RuntimeError; end
-
-  autoload :ExternalRuntime,  "execjs/external_runtime"
-  autoload :MustangRuntime,   "execjs/mustang_runtime"
-  autoload :RubyRacerRuntime, "execjs/ruby_racer_runtime"
-  autoload :RubyRhinoRuntime, "execjs/ruby_rhino_runtime"
-  autoload :Runtimes,         "execjs/runtimes"
-
-  class << self
-    attr_reader :runtime
-
-    def exec(source)
-      runtime.exec(source)
-    end
-
-    def eval(source)
-      runtime.eval(source)
-    end
-
-    def compile(source)
-      runtime.compile(source)
-    end
-
-    def runtimes
-      Runtimes.runtimes
-    end
-
-    def runtime=(runtime)
-      raise RuntimeUnavailable, "#{runtime.name} is unavailable on this system" unless runtime.available?
-      @runtime = runtime
-    end
-
-    def root
-      @root ||= File.expand_path("../execjs", __FILE__)
-    end
-
-    def windows?
-      @windows ||= RbConfig::CONFIG["host_os"] =~ /mswin|mingw/
-    end
-  end
-
-  # Eagerly detect runtime
-  self.runtime ||= Runtimes.best_available ||
-    raise(RuntimeUnavailable, "Could not find a JavaScript runtime. See https://github.com/sstephenson/execjs for a list of available runtimes.")
+  self.runtime ||= Runtimes.autodetect
 end
diff --git a/lib/execjs/module.rb b/lib/execjs/module.rb
new file mode 100644
index 0000000..f1b4d4d
--- /dev/null
+++ b/lib/execjs/module.rb
@@ -0,0 +1,39 @@
+require "rbconfig"
+
+module ExecJS
+  VERSION = "1.0.0"
+
+  class Error           < ::StandardError; end
+  class RuntimeError              < Error; end
+  class ProgramError              < Error; end
+  class RuntimeUnavailable < RuntimeError; end
+
+  class << self
+    attr_reader :runtime
+
+    def runtime=(runtime)
+      raise RuntimeUnavailable, "#{runtime.name} is unavailable on this system" unless runtime.available?
+      @runtime = runtime
+    end
+
+    def exec(source)
+      runtime.exec(source)
+    end
+
+    def eval(source)
+      runtime.eval(source)
+    end
+
+    def compile(source)
+      runtime.compile(source)
+    end
+
+    def root
+      @root ||= File.expand_path("..", __FILE__)
+    end
+
+    def windows?
+      @windows ||= RbConfig::CONFIG["host_os"] =~ /mswin|mingw/
+    end
+  end
+end
diff --git a/lib/execjs/runtimes.rb b/lib/execjs/runtimes.rb
index f99777b..80e11cc 100644
--- a/lib/execjs/runtimes.rb
+++ b/lib/execjs/runtimes.rb
@@ -1,3 +1,9 @@
+require "execjs/module"
+require "execjs/external_runtime"
+require "execjs/mustang_runtime"
+require "execjs/ruby_racer_runtime"
+require "execjs/ruby_rhino_runtime"
+
 module ExecJS
   module Runtimes
     RubyRacer = RubyRacerRuntime.new
@@ -32,10 +38,34 @@ module ExecJS
     )
 
 
+    def self.autodetect
+      from_environment || best_available ||
+        raise(RuntimeUnavailable, "Could not find a JavaScript runtime. " +
+          "See https://github.com/sstephenson/execjs for a list of available runtimes.")
+    end
+
     def self.best_available
       runtimes.find(&:available?)
     end
 
+    def self.from_environment
+      if name = ENV["EXECJS_RUNTIME"]
+        if runtime = const_get(name)
+          if runtime.available?
+            runtime if runtime.available?
+          else
+            raise RuntimeUnavailable, "#{name} runtime is not available on this system"
+          end
+        elsif !name.empty?
+          raise RuntimeUnavailable, "#{name} runtime is not defined"
+        end
+      end
+    end
+
+    def self.names
+      constants
+    end
+
     def self.runtimes
       @runtimes ||= [
         RubyRacer,
@@ -48,4 +78,8 @@ module ExecJS
       ]
     end
   end
+
+  def self.runtimes
+    Runtimes.runtimes
+  end
 end