GirlStars, an after-school health education and physical activity program helps girls aged 9–13 years in public housing stay away from unhealthy behaviors and engage in activities that improve health. The program aims to reduce or prevent smoking, drug and alcohol use, and unsafe sexual activity. Developed in collaboration with the Boston Public Health Commission and the Boston Housing Authority in 2001, GirlStars is provided at two of Boston’s public housing locations. Girls in the program, most of whom are Hispanic or African American, participate in weekly health education classes and physical activities. Researchers are evaluating whether the program improved the girls’ knowledge of health issues, avoidance of risky behaviors, and physical activity levels, and also led to positive changes in the girls’ academic achievement, self-esteem, and well-being.