Reports from 13 Further Mathematics Knowledge Networks supported by the National Centre for Excellence in the Teaching of Mathematics [NCETM] are analysed. After summarizing basic characteristics of the networks regarding leadership, composition and pattern of activity, each of the following aspects is examined in greater depth: Developmental aims and mathematical/pedagogical foci; Professional development activity at network meetings; Perceived impacts of the networks on professional learning and collective organization, and on classroom practice and student response. This provides some support for the claim that involving teachers in collaborative reflection and enquiry pays dividends in producing real results in the classroom. However, these findings rest predominantly on the professional judgement of participants.