The pervasive use of personal mobile devices provides an advantage for the delivery of a behavioral intervention anywhere and anytime. However, behavioral outcomes are dependent on adoption and sustained use of mobile-based interventions. Design approaches for engagement and feasibility of use in everyday life have been studied among the general population, especially to promote physical activity. In this paper, we discuss a design process that addresses additional barriers to adoption when the target population has symptoms, limitations, or needs resulting from a specific health condition. We designed a smartphone-based physical activity intervention for breast cancer survivors by combining behavior change theory with empirical evidence. In a preliminary evaluation, participants found the application engaging and useful, and continued to use the application briefly after the evaluation ended. We illustrate how our design process was used, and could be applied to other specific health conditions.