Research on complex behavior change interventions has largely focused on intervention development and testing their effects in feasibility trials, pilot studies, and randomized controlled trials. However, a significant gap exists in translating behavior interventions informed by theory into real-world practice. This chapter describes how engaging stakeholders can improve the likelihood that effective behavior change interventions are put into practice. The chapter begins with an overview of implementation science and normalization process theory – which outlines how effective interventions are routinely implemented. The roles of stakeholders as research partners and research participants are differentiated using research in health contexts. For example, the process of stakeholder involvement is illustrated using digital health interventions for people with long-term physical health conditions with reference to UK Medical Research Council guidelines on complex interventions. The examples illustrate (1) how stakeholder support in the co-design of complex interventions can improve their utility, usability, accessibility, and acceptability and (2) how stakeholder perspectives elicited using mixed methods during the feasibility and pilot phases of intervention development can help inform subsequent stages of intervention development. Finally, the evaluation and implementation phase is explored, using a case study to illustrate the need to engage with additional stakeholders to translate effective interventions into routine practice.