Most theories on which behavior change interventions are based focus on individual self-regulation and neglect the influence of the social environment. This chapter highlights theoretical approaches and empirical research on effects of the social environment on behavior change with a focus on dyadic behavior change interventions. Dyadic behavior change comprises “parallel” techniques that can be any individual technique directed at both partners in the dyad; “cross-over” techniques that involve interaction between partners but not necessarily administered with both partners present; and “joint” techniques in which both partners are actively involved. A continuum of individual to dyadic behavior change techniques that address different degrees of involvement of dyad members is provided. Based on the continuum, the chapter reviews dyadic behavior change intervention research and the theories involved. The chapter highlights the need for systematic research in dyadic behavior change and more comprehensive reporting of intervention content and delivery. A step-by-step guide provides guidance on effective implementation of dyadic behavior change interventions based on existing theory and current evidence. Owing to the heterogeneity in content, theoretical background, and empirical results, however, the guide also defines the research gaps and required research on dyadic behavior change to address them.