This paper is the text of a presentation to the 1st International Workshop on Complexity and Policy Analysis delivered by Gerald Midgley and transcribed and edited by Kurt Richardson. It charts the development of systems thinking since the 1960s, identifying a number of different systems paradigms. These are then compared with paradigms in complexity research, and significant parallels are identified. It is argued that there are several interacting research communities (including those writing about complexity, systems thinking and cybernetics) that have the potential to learn from one another. A research program on systemic intervention is then presented, focusing on the need to think critically about boundaries and values as a means of dealing with the inevitable lack of comprehensiveness in systemic interventions. A rationale for methodological pluralism is also given. All through the paper, the theoretical and methodological ideas are illustrated with practical examples.