A number of ways of intervening rationally in human affairs are based on systems thinking: understanding real-world complexity via systems ideas. This article reviews the development of systems thinking and focuses on one of the systems approaches: Soft Systems Methodology (SSM). It indicates how SSM embodies systems thinking by giving an account of the crucial steps in the thinking as SSM developed. Issues in current work are reviewed by setting out a number of experientially derived generalizations. These concern: SSM as an ideal-type process that the user suitably adapts on each occasion of use; the fact that SSM's systemicity lies primarily in the process of inquiry; the fact that its focus is the interaction between theory and practice; the implicit belief behind SSM that learning is axiomatically good; and the belief that SSM is best used participatively.