The prevalence of sites in which users can contribute content increases ordinary citizens' participation in emerging forms of knowledge sharing. This article investigates the practices associated with the roles of participants who actively contribute to the coproduction of knowledge in three online communities and how these roles differ in controversial and noncontroversial threads. The Measles, Mumps, and Rubella (MMR) vaccine was selected as a contentious scientific topic because of persistent belief about an alleged link between the vaccine and autism. Contributions to three online communities that engage mothers with young children were analyzed to identify participant roles. No consistent roles were evident in noncontroversial threads, but the role of mediator consistently appeared in controversial threads in all three communities. This study helps to articulate the roles played in online communities that engage in knowledge collaboration. The variety of roles in online communities has implications for both the study for practice and the design of information technologies.