There is increasing interest among policymakers and practitioners in tapping the potential of family, friends, volunteers, peer support groups, and mutual aid organizations to help prevent violence. The popularity of these informal social support (ISS) interventions stems, in part, from their flexibility, responsiveness to individual needs, and perceived low cost. However, there is still limited understanding of whether and how ISS interventions can improve social support, reduce violence, or save money. Furthermore, mobilizing and sustaining ISS interventions appears to be difficult, particularly for families living in high-risk environments. Rigorous and creative evaluations of ISS interventions are needed to inform policy decisions and refine program development and implementation. Focusing on the field of child maltreatment, we describe different kinds of ISS interventions and outline an evaluation agenda that includes core research questions and evaluation challenges and strategies.