An abstract model is developed of "computer-mediated activity" or CMA. The CMA model is based on the existing literature and describes how people interact with software to fulfil an intention and how, over time, they may achieve full mastery of the software. Academic studies related to the design of user documentation for software are sparse, and no underlying reference framework is available within which design rationale could be expressed. The CMA model is intended as a first step towards the development of such a framework. Its implications for documentation design are discussed and its application is illustrated by describing a number of well-known documentation formats in terms of the model.