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. In this monograph, we provide an account of diﬀerent design approaches to common features of mobile wellness applications and we discuss the tradeoﬀs inherent in those approaches. We also outline the key challenges that HCI researchers and designers will need to address to move the state of the art for mobile wellness technologies forward.