This chapter describes key methods to promote intervention engagement in order to maximize uptake, prevent early dropout, and support sustained behavior change. The importance of reviewing or conducting qualitative and mixed methods research on target users’ attitudes, capabilities, and lifestyle is highlighted so that interventions can be designed to meet users’ needs. Tailoring interventions is useful to provide appropriate advice and support for the needs of the target population – especially among those who find it difficult to engage due to personal circumstances or lack of resources. Interventions should then be optimized by collecting data on how people engage with them and iteratively modifying them to improve engagement. Qualitative studies are needed to explore target users’ views of intervention elements. Quantitative usage and outcome data are valuable to analyze usage patterns and identify predictors of dropout or effective behavior change. To maintain longer-term engagement with behavior change, it can be useful to harness social support and establish environment-prompted habits that require less deliberative effort to sustain. The chapter provides examples and tools that can be used to design and optimize interventions, drawing on the “person-based approach” that has been used to develop many interventions that have proved engaging and effective.