We present the syntax, semantics and usage of Abacus, an executable notation for specifying concurrent computations that extends CCS with label prefixing and filtering operators for encapsulating systems of communicating agents and a pattern mechanism for parameterizing behaviour expressions. Abacus is intended to be used as a semantic target and a prototyping tool for the specification of concurent object-based languages and systems. We illustrate the use of Abacus through a series of standard concurrency examples, concluding with an executable specification of SAL, a Simple Actor Language.