This chapter presents a generic, ten-task plan for collaboratively developing, testing, refining, and implementing behavior change interventions. Even carefully designed, expensive interventions can prove ineffective if designers make incorrect assumptions about (1) how a behavior pattern is generated and regulated; (2) how participants or organizations are likely to respond; or (3) how novel personal, interpersonal, or organizational practices can be sustained in situ or over time. Interventionists can be misdirected by assuming that recipients are motivated to change or that motivation is sufficient to evoke change, or that they are choosing reflectively to repeat behavior patterns or that they make choices in an influence-free environment or have just a few everyday life goals. Reliance on any of these assumptions, or the application of simple rules such as educate them, promise rewards, or threaten them, can undermine intervention design from the outset. Interventionists can be effective when they cocreate bespoke, tailored, mechanism-based, interventions that are engaging, rewarding, and sustainable in context. Detailed analyses of mechanism and change processes combined with evidence-based, context-tailored collaborative design is a prerequisite. Success needs to be demonstrable in less expensive efficacy evaluations before investing in large-scale effectiveness trials that provide the evidence base for scaled-up implementation.