The continued creation of online environments and resources for developing teacher professional knowledge has been a major feature of teacher continuing professional development in recent years. Despite the dynamic nature of online environments, professional development is primarily seen as a one-way process of trainers adding to or enhancing teacher knowledge and little is known about the way in which such professional knowledge is used and shared by practitioners within these environments.The Learning Schools Programme was a UK wide professional development programme, which ran from September 1999 to March 2003, and focussed on the integration of ICT across the curriculum. The programme incorporated a number of online elements including an online asynchronous conference environment, FirstClass, with around 50,000 participating teachers.The asynchronous nature of such Computer-mediated Conferencing (CMC) alters the way in which the conference participants can create, use and transform knowledge. Knowledge in this environment becomes cyclical - continually being constructed, used and transformed throughout the life of the conference by the participants, both trainees and advisors, rather than part of a one-way, linear process of knowledge transfer from advisor to teacher. This paper will examine the role that CMC has played in the creation, use and transformation of knowledge by teachers on the Learning Schools Programme. Beginning with the model of professional knowledge proposed by Banks, Leach and Moon (1999), it seeks to explore how this knowledge is created and used in the CMC environment. In particular it will examine how the asynchronous nature of the conferencing environment has affected the lifecycle of knowledge on the conference, transforming such knowledge from a static element to a dynamic, cyclical process of ongoing development within the conference community.Focussing on the three secondary school core subjects of Maths, Science and English, and analysing the posted messages and attachments created in the conference, it intends to examine the form that knowledge takes on the conference, how this knowledge is transferred and transformed by teachers, and to determine what factors affect the life cycle of that knowledge. References: Banks, F., Leach, J. and Moon, B. (1999) 'New understandings of teachers' pedagogic knowledge' In J. Leach, and B. Moon, (eds) Learners and Pedagogy, London, Paul Chapman