This paper draws on data from a larger case study of teacher development to identify what knowledge teachers value most highly in their professional development. The study found that teachers appraise the sources of knowledge available to them through formal, informal, and personal means according to three main criteria: relevance, capacity for involving active engagement, and utility. In general, the teachers found that formal inservice education was less likely than informal collegial exchange or private inquiry to offer opportunities for knowledge that met these criteria. Among the teachers, however, there existed variation in motivation, locus of control, and capacity to process and apply knowledge that influenced the strictness with which individual teachers applied the criteria to assess prospective opportunities for learning. The study concludes that staff developers who respect the criteria that teachers hold for judging the value of knowledge and who assume that it is a professional function of the teacher to create as well as to use knowledge are likely to be highly effective in fostering teacher growth. (Author)