As the rates of lifestyle diseases such as obesity, diabetes, and heart disease continue to rise, the development of effective tools that can help people adopt and sustain healthier habits is becoming ever more important. Mobile computing holds great promise for providing effective support for helping people manage their health in everyday life. Yet, for this promise to be realized, mobile wellness systems need to be well designed, not only in terms of how they implement specific behavior-change techniques but also, among other factors, in terms of how much burden they put on the user, how well they integrate into the user’s daily life, and how they address the user’s privacy concerns. Designing for all of these constraints is difficult, and it is often not clear what tradeoffs particular design decisions have on how a wellness application is experienced and used. Designing for Healthy Lifestyles assesses different design approaches to common features of mobile wellness applications, and discusses the tradeoffs that are inherent in those approaches. It also outlines the key challenges that human-computer interaction researchers and designers will need to address to move the state of the art for mobile wellness technologies forward.