The paper evaluates the role of inquiry in teachers' continuing professional development in the context of 4 projects in which a link was made between developing the critical and creative thinking of learners in the classroom and teacher inquiry. The projects share a common focus on supporting the effective implementation of thinking skills approaches in school settings but differed in terms of the context and interpretation of the role of teacher inquiry. The first project involved teachers participating in the Scottish Chartered Teacher Programme, whilst the Thinking Skills Study Group at the University of Strathclyde focused on the formation of a community of teachers in which learning through scholarship and inquiry is pursued. The North East School Based Research Consortium was originally part of a national funded project to promote the policy of evidence informed practice by encouraging teachers to engage in and with research. The fourth project is linked to economic regeneration through raising learners' aspirations and advocates an approach to professional development based on 'radical encouragement' (Wegerif, 2004). We present four linked case studies of the projects, which include teacher interviews and analysis of project documentation (including papers and articles written by participants). The paper draws upon the perceptions of the teachers participating in each project as to the benefits of being involved and locates the outcomes within theories of the teacher as a researcher and extended professional (Stenhouse, 1975), communities of teachers as learners (Shulman and Sherin, 2004), communities of inquiry (Lipman, 2003) and communities of practice (Wenger, 1998). Problems in establishing relationships between the role of inquiry and evidence of impact on practice in each of the projects are explored and the following questions raised: 1.To what extent is inquiry a necessary as opposed to desirable part of teachers' professional development? 2.What is an appropriate balance between inquiry and learning from others in teachers' professional development? 3.How do we address the problem of scale when developing inquiry based approaches to professional development? Finally, the key issues emerging from the analysis of the projects are discussed in the context of current developments in the promotion of knowledge creating organisations and networks in the UK (Hargreaves, 2004) and the Netherlands (Imants, 2003) and systematic reviews of evidence for effective models of professional development (Cordingley, Bell et al. 2003).