Objective: To develop and test messages and a mobile phone delivery protocol designed to influence the nutrition and physical activity knowledge, attitudes, and behavior of adolescents. Design: Nine focus groups, 4 classroom discussions, and an 8-week pilot study exploring message content, format, origin, and message delivery were conducted over 12 months using a multistage, youth-participatory approach. Setting: Youth programs at 11 locations in Arizona. Participants: Recruitment was coordinated through youth educators and leaders. Eligible teens were 12-18 years old and enrolled in youth programs between fall 2009 and 2010. Phenomenon of Interest: Adolescent preferences for messages and delivery of messages. Analysis: Qualitative data analysis procedures to generate themes from field notes. Results: One hundred seventy-seven adolescents participated in focus groups (n = 59), discussions (n = 86), and a pilot study (n = 32). Youth preferred messages with an active voice that referenced teens and recommended specific, achievable behaviors; messages should come from nutrition professionals delivered as a text message, at a frequency of [less than or equal to] 2 messages/day. Conclusions and Implications: More than 300 messages and a delivery protocol were successfully developed and tested in partnership with adolescents. Future research should address scalability of texting interventions; explore dose associated with changes in knowledge, attitudes, and behaviors; and offer customized message subscription options. (Contains 3 tables.)