Action research has emerged as a particular form of applied social science investigation in attempts to develop useful ways of trying out and evaluating social policy innovations. Despite a variety of contributions to the debate about its methodology, action research remains a term that is employed very vaguely to describe all kinds of design that include some combination of the two elements of action and research. This paper outlines the methods that were adopted in the National Children's Bureau series of action research studies, where the research team attempted to evaluate experiments in which they also actively intervened. The three elements of the field-work setting – programme, process and product – were all important aspects of the research. The inevitable tensions between the demands of evaluation and those of close involvement raise difficult questions of research design. A model of action research is presented as a constructive way of reconciling the search for objectivity and reliability with the advantages of involvement in the definition of objectives and methods.