Social movement scholarship has increasingly sought to understand the relational dynamics of internal movement activity, from investigating the factors that enable movement coalitions to analyzing the trade-offs of organizational hybridity. We bring these and other related phenomena together under the label of boundary-spanning processes. Specifically, we organize research on boundary-spanning in social movements by identifying three types of boundaries that have symbolic significance as categorical demarcations: (a) issue and identity boundaries, (b) organizational boundaries, and (c) tactical boundaries. We then elucidate the tension in work that has examined how each form of boundary-spanning either promotes or hinders the realization of three important movement outcomes: (a) mobilization, (b) internal movement solidarity and scope, and (c) external social and political change. We relate our three types of boundary-spanning to these three types of outcomes in an organizing framework to locate future opportunities for research on boundary-spanning in social movements.