Social media are now used by a majority of American internet users. Social media platforms encourage participants to share information with their online social connections and exchange user-generated content. Significant numbers of people are already using social media to share health-related information. As such, social media provide an opportunity for "user-generated" cancer control and prevention interventions that employ users' behavior, knowledge, and existing social networks for the creation and dissemination of interventions. These interventions also enable novel data collection techniques and research designs that will allow investigators to examine real-Time behavioral responses to interventions. Emerging social media-based interventions for modifying cancer-related behaviors have been applied to such domains as tobacco use, diet, physical activity, and sexual practices, and several examples are discussed for illustration purposes. Despite some promising early findings, challenges including inadequate user engagement, privacy concerns, and lack of internet access among some groups need to be addressed in future research. Recommendations for advancing the field include stronger partnerships with commercial technology companies, utilization of rapid and adaptive designs to identify successful strategies for user engagement, rigorous and iterative efficacy testing of these strategies, and inclusive methods for intervention dissemination. Cancer Epidemiol Biomarkers Prev; 23(9); 1953-6.