The development of information systems (IS) requires methods that recommend how to act during the development process. In some cases existing methods cannot cope with the requirements of the project situation at hand. Therefore, new methods must be developed. Method engineering (ME) attends to this application field. In this chapter, we provide a detailed overview of IS method engineering approaches in order to describe the concept of method engineering. Based on a literature review we derive a formal description of methods that can be used to describe them in a basic way and transfer them to other fields of application. With the formal description of methods this chapter facilitates the process of understanding method engineering both for method user and its engineer.