Message sequence charts (MSCs) constitute an attractive visual formalism that is widely used to capture system requirements during the early design stages in domains such as telecommunication software. A version of MSCs called sequence diagrams is one of the behavioral diagram types adopted in the UML. In this chapter we survey MSCs and their extensions. In particular, we discuss high level MSCs, which allow MSCs to be combined in various regular ways, and the more recent mechanism of communicating transaction processes, which can be used to structure sequence charts to capture system behaviors more directly. We also discuss in some detail live sequence charts (LSCs), a multi-modal extension of MSCs with considerably richer expressive power, and the play-in/out method that makes it possible to use LSCs directly as an executable specification.