The goal of this section is to familiarize the reader with the basic concepts of lexical analysis and particular implementations in quex. First, it is recalled what lexical analysis is all about. Then, it introduces the concept of pattern-action pairs. It also shows, how those pattern-action pairs can be grouped into modes. Next, the concept of incidence handlers is explained. One section provides an overview over all code sections of a lexical analyzer description. A final section provides an overview over use case scenarios and how they are to be handled.