Background Research over the past two decades documents how social capital, or the resources attained through social relationships, is associated with a range of outcomes at both the individual and organizational levels. Yet few, if any, studies explore the relationship between social capital and teaching self-efficacy. Given that teaching self-efficacy is a significant predictor of instructional effectiveness, identifying the kinds of social interactions that facilitate positive teaching self-efficacy can offer important information with respect to how schools and school systems can bolster teachers’ perceived competence and thus their instructional capacity.Objective This study integrates social capital and social cognitive theories to frame an investigation of the social sources that contribute to teachers’ self-efficacy over time. Specifically, we explore how social interactions that vary in their relationship with and proximity to instruction influence teachers’ developing self-efficacy.Research Design We analyzed self-report survey data from 345 teachers in the same district over 4 years. These data captured various social sources of teaching self-efficacy, including indicators of verbal persuasion and vicarious experiences, and allowed us to account for contextual variables found to influence teachers’ sense of mastery. We used the multilevel model for change framework to explore associations between these types of social interactions and teacher self-efficacy over time.Findings Results suggest that interactions firmly rooted in actual teaching practice—namely, those focused on a specific instructional episode (i.e., feedback about a class) or on particular teaching practices (i.e., discussions about specific teaching resources and artifacts)—were associated with higher reports of teaching self-efficacy over time. On the other hand, interactions that reflected more general or less targeted interactions about teaching (i.e., being sought out for general instructional advice or observing someone else teach) were not.Conclusions To further the capacity of individual teachers, school and system leaders should invest in supporting social interactions among teachers that afford direct and targeted opportunities to learn about particular instructional practices and to discuss specific teaching episodes. Interactions less proximal to classroom practice may be ineffective for promoting individual teachers’ feelings of teaching competence.