Some terms have a special importance to the field of systems biology. They cannot be explained exhaustively in a few sentences, which means that any attempt at a real in-depth explanation would by far exceed the format of a glossary. To accommodate their special position and at the same time stay within the chosen format for the glossary, the following chapter has been added. It provides a more exhaustive discussion of the central terms system, model, modularity, and systems biology itself. There are still glossary pages for these terms as well, though, in Chapter 3.