Message Sequence Charts (MSCs) are an attractive visual formalism widely used to capture system requirements during the early design stages in domains such as telecommunication software. It is fruitful to have mechanisms for specifying and reasoning about collections of MSCs so that errors can be detected even at the requirements level. We propose, accordingly, a notion of regularity for collections of MSCs and explore its basic properties. In particular, we provide an automata-theoretic characterization of regular MSC languages in terms of finite-state distributed automata called bounded message-passing automata. These automata consist of a set of sequential processes that communicate with each other by sending and receiving messages over bounded FIFO channels. We also provide a logical characterization in terms of a natural monadic second-order logic interpreted over MSCs. A commonly used technique to generate a collection of MSCs is to use a Message Sequence Graph (MSG). We show that the class of languages arising from the so-called locally synchronized MSGs constitute a proper subclass of the languages which are regular in our sense. In fact, we characterize the locally synchronized MSG languages as the subclass of regular MSC languages that are finitely generated. ∗ Basic Research in Computer Science, Centre of the Danish National Research Foundation.