Project Summary Childhood is a critical period for primary prevention for unhealthful behaviors associated with many diseases later in life. Existing prevention interventions for children, however, rarely incorporate approaches that tailor intervention strategies to disease-specific risk factors. In the prevention of skin cancer, targeting interventions to children is critical as children who incur higher UV exposures are at greater risk of developing skin cancer as adults and sun safety practices learned at young age tend to become lifelong habits. Available skin cancer interventions for children have not been tailored to individual risk factors. Prevention can greatly reduce associated morbidity and mortality for patients and economic burden to the health care system. Sun exposure is an established environmental cause of melanoma and non-melanoma skin cancer, and both are preventable by reducing exposure to ultraviolet (UV) radiation achieved through sun protective behaviors. Individually-tailored intervention strategies that can lead to improving and maintaining sun protective behaviors have not been tested among children. Thus, we aim to develop and pilot test a novel method of mobile and wearable technologies, just-in-time adaptive intervention (JITAI) tailored to children’s individual and real-time risk factors, focusing on skin cancer prevention. In the proposed study, we will 1) develop a prototype of mobile and wearable JITAI, tailored to children’s individual risk factors via co-design process involving children and parents; 2) pilot test usability and feasibility of the prototype JITAI among target users; 3) incorporate intensive longitudinal, self-report sun protection behavior measures and automatically collected environmental sensor data; and 4) collect preliminary data for protocols (engagement strategies, frequency and length) of UV-exposure triggered, context-sensitive, JITAI most feasible for school-aged children. The proposed intervention strategy will enable us to build a platform for a cost-effective, highly scalable approach, receptive to children users that allow for assessment of their own behaviors and environment in real time. The proposed methods are demonstrative and could be applied to a wide variety of other environmental exposures and related health behavior outcomes.