Applied systems thinking has evolved since the 1950s through three paradigmatic waves. Authors in the first wave regarded systems as real-world entities, and systems models as representations of reality, so objectivity was important. In contrast, second wave authors emphasized thinking in terms of systems, and the exploration of multiple perspectives. The role of models was to aid mutual under- standing and enhance the appreciation of diverse viewpoints on possible actions to be taken. In the 1980s, first and second wave advocates came into conflict. Then some third wave authors, initially working under the banner of critical systems thinking, argued that the division of the systems research community into two camps was unhelpful, and they advocated methodological pluralism – mixing methods from both traditions. Other authors set out to address power relations during interventions – in particular, the practice of exploring value and boundary judgments in projects in order to address conflict and marginalization. This practice came to be called “boundary critique,” and it was eventually integrated with methodological pluralism in a new approach called “systemic intervention.” This chapter gives readers a thorough overview of the emergence and maturation of both critical systems thinking and systemic intervention, illustrated with practical exam- ples. It then discusses two major problems that remain unaddressed in the third wave. First, the increasing proliferation of methodologies and methods has resulted in such a diversity of views on systems thinking, that explaining what it is to newcomers has become a real challenge. Second, despite this diversity, all the new methodologies and methods are still founded on principles of rational analysis, and approaches that go beyond this are marginalized. For instance, arts-based and theater methods are rarely mentioned in the literature on systems thinking, yet they can help people discover how their value and boundary assumptions have roots in unconscious impulses and memories. Such discoveries help to unfreeze taken-for-granted understandings, including the internalization of oppressive power relationships. Very recent writings have begun to tackle these problems, but it is too soon to judge whether they represent an extension of the third wave, or the first swellings of a new, fourth wave of systems thinking.