Networks of practice (NOPs) are social spaces where individuals working on similar problems self-organize to help each other and share perspectives about their occupational practice or common interests, and exist primarily through computer-mediated exchange. This exchange of knowledge through message postings produces an online public good, where all participants in the network can access the knowledge, regardless of their contribution. Thus, this research builds upon theories of collective action and public goods to better understand the provision and maintenance of knowledge in a network of practice. We use social network analysis to examine the following research questions: (1) What is the pattern of contribution that produces and sustains the public good? (2) Are NOPs maintained by a critical mass? (3) How does the heterogeneity of resources and interests of participants impact the collective action of the NOP? We find that the network of practice is sustained through generalized exchange, is supported by a critical mass of active members, and is shaped as a star. The critical mass is significantly related to tenure in the occupation, expertise, availability of local resources, and desire to enhance oneís reputation.