The relationship between knowledge and practice has been a long standing issue for social work. This is, in part, because, despite the strong intuitive appeal of a position which emphasises the importance of knowledge, this has yet to be established through research. Part of the problem lies with different epistemological positions, and the consequent differences in the examination of knowledge application. Recently, however, there has been a growth of theoretical interest in reflection, reflexivity and cognitive processes as a way of looking at social work knowledge. This process knowledge represents a break from the past where, despite other differences, writers emphasised 'knowledge as product', that is given knowledge, already researched and available for practice use. Despite impressive theoretical developments, however, process knowledge has not been subject to empirical research. This paper reports on work which sought to develop categories and concepts for process knowledge based on research with social workers. Using a novel method, the cognitive processes interview, a number of categories of process knowledge were identified related broadly to areas of critical appraisal, hypothesis generation and the relationship between the two. The paper provides clear evidence of the need for high level cognitive abilities for the conduct of practice, and the need to develop educational processes which will encourage rigour in relation to these cognitive processes.