Critical systems thinking is a relative newcomer in the systems tradition of thought. Nevertheless, it has already made a number of significant contributions to the field and is now developing more quickly than any other part of systems thinking. The paper charts the origins and nature of this evolving, critical systems, body of work. The author's own impressions of its development are first set out. This helps establish that critical systems thinking has come to rest upon five “commitments” which define its essential character. These commitments are then used to classify the published literature associated with critical systems thinking. The result is a review of the origins and nature of critical systems thinking up to 1990.