The purpose of this study was to examine reasons for teacher participation in online communities of K–12 teachers. The authors interviewed 23 teachers from three self-generated online communities and analyzed more than 2,000 postings in those communities. The findings indicated five reasons for participation: (a) sharing emotions, (b) utilizing the advantages of online environments, (c) combating teacher isolation, (d) exploring ideas, and (e) experiencing a sense of camaraderie. In conclusion, the findings imply that when designing teacher professional development programs, more emphasis needs to be placed on teachers’ emotional sharing and promotion of self-esteem. (Keywords: online communities, teacher professional development, community of practice, teacher emotional sharing.)