).This study examined how content characteristics of antitobacco messages affect smokers' selective exposure to and social sharing of those messages. Results from an experiment revealed that content features predicting smokers' selection of antismoking messages are different from those predicting whether those messages are shared. Antismoking messages smokers tend to select are characterized by strong arguments (odds ratio = 2.02, P = .02) and positive sentiments (odds ratio = 3.08, P = .03). Once selected, the messages more likely to be retransmitted by smokers were those with novel arguments (B = .83, P = .002) and positive sentiments (B = 1.65, P = .005). This research adds to the literature about the content characteristics driving the social diffusion of antitobacco messages and contributes to our understanding of the role of persuasive messages about smoking cessation in the emerging public communication environment. J Natl Cancer Inst Monogr 2013;47:182-187 Cigarette smoking remains the leading preventable cause of mortality in the United States and is a significant cancer risk factor. More than 90% of lung cancer is attributable to smoking tobacco, and tobacco use significantly increases the risk of developing all of the most commonly diagnosed forms of cancer (1). Thus, tobacco control efforts are critically important for cancer prevention. Tobacco control programs have historically relied heavily on traditional mass media to spread antitobacco messages (2-4). But Web 2.0 and the social media have resulted in rapid, profound changes in the media landscape (5-7). Implications for cancer prevention and tobacco control are just beginning to be understood.Rapid diffusion of new information technologies has created unprecedented opportunities for exposure to and sharing of antitobacco-related messages (ie, messages designed to prevent or curtail tobacco use). As of 2012, 81% of adults in the United States use the Internet, and 59% said they looked online for health-related topics (5). Of online adults, 67% reported using social networking sites such as Facebook or Twitter (8). According to a recent survey (9), 53% of US adults get news forwarded to them via e-mail or social networking sites; 36% share news with their social networks via those platforms. In sum, the production and dissemination of health information in general, and of antismoking information in particular, is no longer in the hands of a few experts or major corporations; individuals now actively seek out, generate, and share information across their social networks (7,10).For more than 2 decades, tobacco control researchers have examined how exposure to televised antitobacco advertising affects individuals' cigarette smoking attitudes and behaviors (2,(11)(12)(13)(14). Although television advertising remains a critical component of tobacco control campaigns, it is becoming increasingly important to measure and understand the impact of antitobacco-related information across digital media platforms and the interactions across these platforms...