Systems development methods are used to express and communicate knowledge about systems and software development processes, that is, methods encapsulate knowledge. Since methods encapsulate knowledge, they also encapsulate rationale. Rationale can, in this context, be understood as the reasons and arguments for particular method prescriptions. In this chapter, we show how the combination of two different aspects of method rationale can be used to shed some light on the communication and apprehension of methods in systems development, particularly in the context of tailoring of methods to suit particular development situations. This is done by clarifying how method rationale is present at three different levels of method existence. By mapping existing research on methods onto this model, we conclude the chapter by pointing at some research areas that deserve attention and where method rationale could be used as an important analytic tool.