To be effective and sustainable, HIV-prevention interventions need to be sufficiently powerful to counteract prevailing social norms and diffuse through the targeted community to provide social reinforcement for behaviour change. Social structural and environmental factors are major influences on HIV-related behaviours yet the dearth of conceptualization and operationalization of these factors impede progress in intervention development. In this paper we propose a social ecological perspective to intervention and highlight relevant theories from social psychology and organizational behaviour literatures. We examine social networks and social settings as micro-structural and environmental influences on HIV risk behaviours, social identities and norms, and as important targets for HIV-prevention intervention. Intervention approaches are proposed that target networks and behavioural settings and provide participants with socially meaningful and rewarding behavioural options that are consistent with valued prosocial identities or roles. Examples are presented on how such an approach has been utilized in prior HIV prevention interventions, including our social network-oriented intervention that trained disadvantaged former and current illicit drug users to conduct peer outreach. We describe how behavioural interventions may enhance or introduce new prosocial identities and social roles, and that network members may confer social approval to reinforce these identities and roles, leading to sustained behavioural risk reduction and changes in risk behaviour norms.