How do members’ and leaders’ social network structures help or hinder team effectiveness?  A meta-analysis of 37 studies of teams in natural contexts suggests that teams with densely configured interpersonal ties attain their goals better and are more committed to staying together; that is, team task performance and viability are both higher.  Further, teams with leaders who are central in the teams’ intragroup networks and teams that are central in their intergroup network tend to perform better.  Time sequencing, member familiarity, and tie content moderate structure-performance connections.  Results suggest stronger incorporation of social network concepts into theories about team effectiveness.