What is health? How do we improve it? Why should we improve it? These are questions peo- ple don’t think much about until they become “unhealthy”. An increasing number of personal health technologies are being designed, which help people collect and reflect on their health and wellness. These technologies are components that can support the prevention, diagnosis and treatment of health and wellness issues. The thesis is focusing on these personal health technologies, and the research is situated in the MONARCA project, where a system support- ing the treatment of bipolar disorder have been designed, implemented and clinically tested. The contributions include both a novel system, the outcomes of the clinical trials reporting of the usability and usefulness of the system, as well as findings on how the technology improves the actual treatment, all based on the included six papers. This thesis further provides con- ceptual contributions in terms of perspectives with which to guide the design of new personal health technologies. It identifies four core design elements of personal health technologies, as well as three design targets for an improved management of health. The contributions should be of interest to researchers working with personal health technologies in HCI, Ubicomp and in clinical research, as well as to practitioners and designers tasked with designing and imple- menting new types of personal health technologies. More broadly, this dissertation also has implications for the construction of sensing and feedback technology in general, including domains such as pervasive health, health behavior change and personal informatics.