One method in which health professionals can influence adolescent substance use is by correcting social norms. According to the social norms theory, individuals often erroneously believe that the majority of people engage in unhealthy behaviors, a misperception that may motivate individuals to increase their own unhealthy behaviors. The social norms theory argues that when misperceptions regarding the substance use of peers are corrected, people will tend to lower their own substance use. Teachers can address the social norms theory in the classroom setting by utilizing an activity known as the "Snowball Survey." Developed by Mary Crozier and later adapted by Linda Hancock, the Snowball Survey is becoming a popular teaching technique for changing the social norms of students. Creators of "Crossroads," a high school substance use prevention program implemented through the University of North Carolina Wilmington, have successfully coupled the Snowball Survey with a student-led social norms poster campaign. Both the Snowball Survey and the poster campaign used in the Crossroads program are described in this article. (Contains 2 figures.)