This chapter describes the development of critical systems thinking, its basic themes, and a practical application that had an effect on the lives of millions of people. The chapter presents the way critical systems thinking emerged, as well as an account of some of the important contributions that it produced in the last decades. Its two key cornerstones are discussed: the emancipatory-oriented strand and the pluralism-oriented strand. Subsequently, the basic themes or commitments of this field are presented, and an instance of a real life implementation that involved the participation of a wide variety of stakeholders and communities. This application illustrates some of the main themes and principles of critical systems thinking, as well as the significant impact it can have in the wellbeing of societies and organisations. Finally, some future potential developments of critical systems thinking are presented, showing its dynamism and capacity to transform the systems approach theory and practice.