Health-related information collected in psychological laboratories may not be representative of people's everyday health. For at least 70 years, there has been a call for methods that sample experiences from everyday environments and circumstances. New technologies, including cell phones, sensors, and monitors, now make it possible to collect information outside of the laboratory in environments representative of everyday life. We review the role of mobile technologies in the assessment of health-related behaviors, physiological responses, and self-reports. Ecological momentary assessment offers a wide range of new opportunities for ambulatory assessment and evaluation. The value of mobile technologies for interventions to improve health is less well established. Among 21 randomized clinical trials evaluating interventions that used mobile technologies, more than half failed to document significant improvements on health outcomes or health risk factors. Theoretical and practical issues for future research are discussed.