Conflict and cooperation do not result from isolated individual actions. In settings such as insurgency, interstate conflict, protest mobilization, and informal governance, actors are highly interdependent. The study of networks aims to identify what the relevant interdependencies are and, crucially, how they shape conflict and cooperation outcomes. Although this is a relatively new research area, its early results convincingly establish that networks matter. Social networks provide information, transmit peer pressure, and structure interactions in ways that help groups overcome social dilemmas. With much research documenting the importance of particular outcomes in particular areas, the next major step will be putting the pieces together. Which connections between actors matter in which circumstances and how? The groundwork has been well laid for this large future research endeavor. Expected final online publication date for the Annual Review of Political Science, Volume 24 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.