The LSD notation is intended for the specification and description of communicating systems of sequential processes. It was originally developed with a semantic model for the CCITT standard language SDL in mind . Like SDL, LSD is primarily conceived as a medium for describing systems at the higher levels of abstraction. A central idea of LSD is the integration of functional and procedural models for synchronisation mechanisms at different levels of abstraction. This integration is based upon the "generalised spreadsheet" or definitive notation paradigm for programming (,), and the operational semantics of LSD is defined in terms of dialogue states over such a notation. The paper includes an informal introduction to the notation, together with several illustrative examples. A methodology for the top-down development of LSD specifications for communicating systems is proposed.