The thesis has its origins in a concern that teacher librarians in schools throughout Australia were disadvantaged in the development of their professional learning due to their professional and often geographical isolation in schools. A listserv (online discussion network) called OZTL_NET was developed to facilitate the enhancement of teacher professional learning for this group of teachers. OZTL_NET has been available to teacher librarians and others interested in teacher librarianship for over nine years. The study sought to determine whether usage of OZTL_NET was associated with the enhancement of teacher librarians’ professional learning. The study also explored the characteristics of teacher professional learning from the literature and sought to determine which characteristics of online communities may contribute to teacher professional learning. A case study design for the research was adopted using a mixed methods approach. The methods of data collection were a web survey and semi-structured interviews. Quantitative and qualitative data were collected and analysed and a detailed description of the object of study, OZTL_NET, was provided. A major finding of this study was that usage of OZTL_NET was significantly related to the enhancement of teacher professional learning. It was concluded that online learning communities may provide appropriate contexts for teacher professional learning and that, in relation to the case of OZTL_NET, various aspects of the concept of online learning communities and, to a lesser extent, communities of practice, are portrayed in the listserv. The findings supported the assertion that online learning communities have the potential to enhance the professional learning of teachers and provide opportunities for teachers to learn online. The study also revealed that community building online is a complex and demanding activity. Usability and sociability factors must be carefully considered and developed over the lifetime of the community. This process should include input from the community, the leadership of which should be broad-based and inclusive. Two broad principles emerged from the research that provide guidance for the management of listservs for teacher professional learning online. First, the study revealed that involvement and collaboration were critical ingredients in teacher professional learning online. Involvement was portrayed not only in the learning that ensued from the interactivity that necessarily underpins the sharing of tacit knowledge through information exchange and professional discussion between and among subscribers online but also by individuals through less obvious means such as lurking, archive searches and off-list communications. In terms of collaboration, this study found that in addition to high levels of trust, subscribers experienced a strong sense of collegiality and support as members of OZTL_NET. Second, the major finding above confirms that involvement and collaboration are strongly related to individual and collective orientations of teacher professional learning. The broad concept of individual or collective orientation recognises that teacher professional learning occurs in both orientations online as it does offline. Teachers have long recognised their colleagues as their major source of professional information. The difficulty in the past has been in the identification of a means by which teachers can readily access a wider pool of colleagues with whom they can discuss important issues, seek advice and so on. This is particularly important for teachers who are professionally isolated as a consequence of their teaching speciality (such as teacher librarians) and those who are geographically isolated making real time meetings with colleagues expensive and/or impracticable. In this context listservs such as OZTL_NET can play a critical role in providing the infrastructure to support distributed models of teacher professional learning online.