This paper is concerned with the construction of mathematical models to represent the action of drugs or other stimuli on biological organisms or other complex systems. A framework for a general system of models is set up on the basis of three main concepts: the system and its subsystems, the sites of action, and the components of the stimulus. The classification of joint action in terms of this framework is then considered and a clarification of the standard nomenclature in relation to dependent joint action is put forward. A more specific set of models, based upon a simplified representation of drug-receptor action, is developed within the general framework. Some practical examples of joint action in the context of the models are described.