Speech act theory is important not only in Linguistics, but also in Computer Science. It has applications in Distributed Computing, Distributed Artificial Intelligence, Natural Language Processing, and Electronic Data Interchange protocols. While much research into speech acts has been done, one aspect of them that has largely been ignored is their semantics, i.e. their conditions of satisfaction. A formal semantics for speech acts is motivated and presented here that relates their satisfaction to the intentions, know-how, and actions of the participating agents. This makes it possible to state several potentially useful constraints on communication and provides a basis for checking their consistency.