Systems analysis, a method originating in engineering and planning areas, evaluates the costs and consequences of alternative approaches to a goal given a known set of constraints. Mental health services are badly in need of a more rational approach to planning, and this article shows how specific systems analysis techniques can be used to describe a mental health system and, at the same time, to evaluate consequences and costs of changing the system in different ways. The limitations of the approach are also considered. The authors' main aim is a general introduction to the systems approach for mental health practitioners.