Research on teachers' professional development is gaining popularity among educators since changes in society require teachers to improve their skills and knowledge. Rather than transmitting information to teachers, knowledge sharing through emerging tacit knowledge among them has gained more importance. Because of new information and communication technologies, knowledge sharing among educators is becoming easier. This article examines the relationship among three important topics; teachers' professional development, knowledge management and online communities of practice. Furthermore, some online learning communities of practice environments are introduced. (Contains 6 footnotes, 2 figures, and 2 tables.)