Digital technology has the potential to transform health care, and therefore health psychology. This chapter first considers user-initiated engagement with digital technology, describing patterns and theories of online health-information seeking and participation in user-generated sites. Next, the chapter provides an introduction to the development of digital behavior change interventions, briefly reviewing models proposed to guide intervention design. The importance of user-centered approaches to development is highlighted; these draw on participatory and qualitative methods to overcome barriers to engagement and maximize impact on behavior change. The implementation of digital interventions is then discussed, focusing on the potential for global low cost dissemination and the challenges that must be overcome to achieve this. Novel methods of using quantitative evaluation and optimization methods are presented, and the chapter concludes with a reflection on future directions for digital health psychology and the need to build new multidisciplinary ways of working to fully exploit the exciting technological possibilities for evaluating and supporting health-related behavior.