Introduction: Critical systems heuristics (CSH) draws on the substantive work and philosophy of C. West Churchman, a systems engineer who, along with Russell Ackoff during the 1950s and 1960s, defined operations research in the United States. Churchman later pioneered developments in the 1970s of what is now known as 'soft' and 'critical' systemic thinking and practice in the domain of social or human activity systems. Churchman died in 2004. His legacy lies in signalling the importance of being alert to value-laden boundary judgements when making evaluations. Boundaries are what we socially construct in designing and evaluating any human activity system of interest (e.g., any situation of concern from a kinship group, an organisation, or a larger entity such as a national health system). The primary boundary of any human activity systems is defined by 'purpose'. Churchman's work is characterised by a continual ethical commitment to the overarching purpose of improved human well-being. In order to fulfil such purposeful activity, there is always a need to broaden inquiry from the particular system of focus so as to appreciate what Churchman calls the total relevant system. The effectiveness and efficiency of a system of interest depends on the actual boundary judgements associated with that system of interest. Churchman first identified 9 conditions or categories (including the category 'purposeï¿½) associated with any purposeful system of interest in his book The Design of Inquiring Systems [1, 2]. He later extended these to 12 categories in a book provocatively entitled The Systems Approach and Its Enemies, significantly taking into account 3 extra factors (ï¿½enemiesï¿½) that lie outside the actual system of interest but which can be affected by, and therein have an effect on, the performance of the system [1, 2]. In the early 1980s a doctorate student of Churchman from Switzerland, Werner Ulrich, translated Churchman's 12 categories into an operational set of 12 questions which he called critical systems heuristics . Ulrich returned to Switzerland and worked with CSH as a public health and social welfare policy analyst and program evaluator . Section 2 introduces the basic toolbox of CSH, along with suggestions on when to use it and the benefits of its use. Section 3 will guide you through a suggested operational use of CSH questions in a process of evaluation. Section 4 provides a summary of an extensive case study in which CSH was used for evaluating the role of public participation in natural resource-use planning. Section 5 provides some advice for the practitioner in developing skills on using CSH for evaluation.