In mature sciences, building theories is the principal method of acquiring and accumulating knowledge that may be used in a wide range of settings. In software engineering, there is relatively little focus on theories. In particular, there is little use and development of empirically-based theories. We propose, and illustrate with examples, an initial framework for describing software engineering theories, and give advice on how to start proposing, testing, modifying and using theories to support both research and practise in software engineering.