The study of systems in themselves, usually to find characteristics common to all systems or to classes of systems. Systems theorists may be most concerned with the development of theory for its own sake (most often called general systems theory), or they may be more concerned with the applications of systems ideas within particular disciplines or problem areas in order to solve problems that are not amenable to traditional ‘reductionist’ approaches. Systems theory has been called the study of organized complexity.

There have been a number of attempts to categorize systems. Perhaps the simplest and most useful is by P. Checkland, who proposes four categories: natural systems, designed physical systems, designed abstract systems, and human activity systems. He also proposes four concepts that are central to systems thinking:

the notion of whole entities which have properties as entities (emergent properties …); the idea that the entities are themselves parts of larger similar entities, while possibly containing smaller similar entities within themselves (hierarchy …); the idea that such entities are characterized by processes which maintain the entity and its activity in being (control …); and the idea that, whatever other processes are necessary in the entity, there will certainly be processes in which information is communicated from one part to another, at the very minimum this being entailed in the idea ‘control’.