This Notice of Funding Opportunity (NOFO) invites R34 applications to support the planning activities necessary to develop social network interventions to promote health across the lifespan, especially in populations in which they are currently largely underdeveloped and untested (such as populations in mid- and late-life). Applications suited to this R34 will focus on planning activities for social network interventions for which a target interpersonal process of behavior change or social network characteristic has already been identified. Planning activities are those activities that are expected to yield necessary and sufficient information to inform final decisions about a social network health behavior change intervention prior to instigation of a hypothesis-driven trial to test a social network intervention. Activities may include (but are not limited to) team-building, protocol development, piloting of systems for data collection and/or management, feasibility and acceptability testing, staff training, and establishing documentation procedures. Basic experimental and observational behavioral and/or social science applications that test how intrapersonal and interpersonal mechanisms of behavior change interact with, influence, or are influenced by characteristics of social networks are outside the scope of this RFA. Research to identify targets for future social network health behavior change interventions and/or to develop, refine, or optimize measures (assays) of putative targets are also outside the scope of this RFA. Both of these types of research are most appropriate for the companion R01 RFA (RFA-AG-24-025). Projects that propose to test the efficacy or effectiveness of a social network health behavior change intervention are also outside the scope of this RFA, which focuses only on pre-trial planning and development activities.