The concept of communities of practice (CoP) was first introduced by Lave and Wenger in their short monograph, "Situated Learning: Legitimate Peripheral Participation." Since then, the concept has become immensely popular in a variety of disciplines. This dissertation traces the concept of CoP back to its inception and early transformation at the Institute for Research on Learning (IRL) and unveils various interpretations. CoP is usually understood as a group of people bound together by their interest in a common working practice and has connotations, such as warm, closed, unchanging, harmonious, and homogeneous. However, the original intent of CoP was lost when the concept became popular. In particular, this dissertation re-examines the notions of contradiction and power, closely associated with legitimate peripheral participation, which is the central concept of Lave and Wenger's monograph. The main objective of this dissertation is to examine whether CoP can still be an incisive analytical tool in understanding professional communities whose landscape is dramatically changing. Relatedly, the dissertation will examine if CoP is useful in analyzing new types of virtual communities, such as arise in a Web 2.0 world. The research relies on two field studies of professional communities: (1) a community of knowledge management practitioners and (2) a blog forum of special librarians in a geographically dispersed corporation. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]