Self-management interventions for long-term conditions may improve the quality of life. Psychosocial techniques can be delivered via digital technologies such as smartphone applications. Interventions should be useful and meaningful to users, but also theory- and evidence-based. This chapter explores some of the processes and challenges involved in developing self-management interventions using co-design, theory, and evidence. Drawing on app development research focused on supporting those with Sjögren’s syndrome (an autoimmune and rheumatic disease with complex symptom patterns), the chapter discusses the practical difficulties we had in generating and respecting individuals’ ideas, preferences, requirements, and creativity, while acknowledging the need for an intervention to include established cognitive components, behaviour change techniques, and “active ingredients”. The chapter is organised around three tensions from our project where our ambition to co-design the intervention with participants was challenged and contradicted. In such situations, we often found ourselves having to judge the relevancy or compatibility of participants’ viewpoints and lived experiences against the evidence base. We close with critical reflections on these tensions, discussing the potential limits of co-design and user-led approaches to creating health and care interventions in a disciplinary culture of evidence-based practice. We offer a series of implications for conducting design research in the area of self-managing long-term conditions, identifying ways for the competing paradigms of co-design and evidence-based design to be bridged.