We outline a theory of communicating “Abstract State Machines”. The state of an Abstract State Machine has two components: a behavioural state and a data state. The behavioural states are shown on a state diagram, whose transitions are labelled with an “event” and a B operation. The firing of a transition is synonymous with the occurrence of its associated event. We use a synchronous model of communication based on shared events which simultaneously change the state of each participating machine. The B operation associated with a transition generally has the form G ⟹ S, where a necessary condition for the transition to fire is that G is true, and where S describes any resulting changes in the data state of the Abstract Machine. The paper includes simple examples, the translation of Abstract State Machines to B Action Systems, the translation of Abstract State Machines into “primitive” Abstract State Machines which have only behavioural state, the parallel combination of high level Abstract State Machines, and short notes on choice and refinement.