The MeDUSA reference manual second edition MeDUSA (Method for UML2 based Construction of Embedded & Real Time Software) is a model based software construction method targeting the domain of small embedded & real time software. MeDUSA was developed by the Research Group Software Construction of the RWTH Aachen University in close cooperation with the German ABB Corporate Research Centre. It incorporates various practical experiences gained during the industrial development of embedded software in ABB Business Unit Instrumentation. Being Use Case driven MeDUSA systematically covers the software construction lifecycle phase from the early requirements up to implementation. Models are successively developed and employed throughout all activities. By enforcing a class based rather than an object oriented design a smooth transition of the resulting design model towards an implementation in a procedural programming language is facilitated. This is essential as procedural programming languages as the C language are still state of the art in the domain of small embedded & real time software.