In 2012, the National Institute of Justice’s (NIJ) Domestic Radicalization to Terrorism program began funding research on issues related to domestic radicalization and terrorism in the United States. NIJ-sponsored research has addressed a variety of factors that can play a role in both facilitating and preventing domestic radicalization, including social networks. This report synthesizes findings from that research, focusing on the role that peer relationships and social dynamics can play as either a gateway to or a gatekeeper preventing radicalization in the United States.