Background: As evidence grows about the benefits of policy and environmental changes to support active living and healthy eating, effective tools for implementing change must be developed. Youth advocacy, a successful strategy in the field of tobacco control, should be evaluated for its potential in the field of obesity prevention., Community Context: San Diego State University collaborated with the San Diego County Childhood Obesity Initiative to evaluate Youth Engagement and Action for Health! (YEAH!), a youth advocacy project to engage youth and adult mentors in advocating for neighborhood improvements in physical activity and healthy eating opportunities. Study objectives included documenting group process and success of groups in engaging in community advocacy with decision makers., Methods: In 2011 and 2012, YEAH! group leaders were recruited from the San Diego County Childhood Obesity Initiative's half-day train-the-trainer seminars for adult leaders. Evaluators collected baseline and postproject survey data from youth participants and adult group leaders and interviewed decision makers., Outcomes: Of the 21 groups formed, 20 completed the evaluation, conducted community assessments, and advocated with decision makers. Various types of decision makers were engaged, including school principals, food service personnel, city council members, and parks and recreation officials. Eleven groups reported change(s) implemented as a result of their advocacy, 4 groups reported changes pending, and 5 groups reported no change as a result of their efforts., Interpretation: Even a brief training session, paired with a practical manual, technical assistance, and commitment of adult leaders and youth may successfully engage decision makers and, ultimately, bring about change.