Because it is not clear that teachers will volunteer for good professional development if it is made available, this study used surveys--including social network analysis--and interviews to examine why teachers did and did not participate in one school's high-quality study group. While all teachers were motivated by intrinsic incentives, two factors separated participators from nonparticipators. Nonparticipators were more susceptible to barriers like time constraints and personal factors. Participators had more social incentives. They engaged more actively in friendship and advice networks, especially with other participators. Practical implications are suggested. [Paper presented at the Annual Meeting of the American Educational Research Association in Chicago, Illinois, April 17, 2015.]