Although largely unrecognized, diverse theories of crime provide insight into an important source of deterrence: the role community members as informal agents of deterrence. This issue is explored in three sections. The first section argues that as social disorganization theory evolved into collective efficacy theory, scholars in this tradition have placed increasing emphasis on community members exerting direct informal control in the community. The willingness of the public to intervene to protect social order has been conceived, in effect, as a key ingredient in the deterrence of crime. The second section reviews the contributions of environmental criminology, a perspective that has focused explicitly on the role of community members in deterring would-be offenders through what is referred to as “guardianship.” The third section presents theoretical models (i.e., broken windows, code of the street, and legal cynicism) that identify conditions that undermine the capacity of community members to deter. Deterrence is seen to be less effective when residents lose control of public spaces and have weak relationships with the formal agents of control, especially the police. The chapter concludes by deriving a set of propositions that might form the beginning basis of a theory of community members’ deterrence.