The global epidemic of obesity is staggering in its scope. The World Health Organization (2011) reports that nearly 1.5 billion adults aged 20 years and older and 43 million children younger than 5 years are overweight. The health, societal, and economic impact of obesity on current and future generations is daunting. Obesity is associated with an increased prevalence of serious chronic diseases and con-ditions, including cardiovascular diseases, diabetes mellitus, cancer, musculoskeletal conditions, mental health disorders, and physical function limitations and disability (Abdullah et al., 2011; Alley & Chang, 2007; Deaton et al., 2011; Martin, Freedman, Schoeni, & Andreski, 2010; Wang, McPherson, Marsh, Gortmaker, & Brown, 2011).Obesity among participants 30 to 49 years old in the Framingham Study was associated with a doubling of proba-bility of having a physical function limitation in older age (Peeters, Bonneux, Nusselder, De Laet, & Barendregt, 2004). Available data on direct health care costs estimate that obesity accounts for 74 billion dollars of all health care expenditures in the United States and 33 billion euros in the European Union and 25% of all health expenditures in Canada (Allender, Foster, Scarborough, & Rayner, 2007; Anis et al., 2010; Trasande & Elbel, 2012). An additional 5.5 billion British sterling pounds in health care expenditures associated with obesity are projected by 2050 in the United Kingdom (Trasande & Elbel, 2012; Wang et al., 2011).Physical inactivity and poor dietary habits are the primary contributors to overweight and obesity. Individually, physi-cal inactivity and poor dietary habits increase the risks of developing many chronic diseases and conditions—and these risks are compounded in the presence of obesity (U.S. Department of Health and Human Services, 2008; World Health Organization, 2011, 2012). Physical inactivity and unhealthy dietary habits are highly prevalent in developed and—increasingly so—in developing countries throughout the world (World Health Organization, 2008, 2011).The body of scientific literature about physical activity and diet has grown exponentially over the past decade, demon -strated by the sheer volume of published articles in this and other journals. However, the effectiveness of long-term interventions to promote physical activity has been equivo-cal, but shorter term interventions have shown some success in improving physical activity (Garber et al., 2011). Obesity prevention and treatment programs have been effective in children (Oude Luttikhuis et al., 2009; Waters et al., 2011) but somewhat less so in adults (Appel et al., 2011; Wing & Phelan, 2005). Thus, there remains much to learn about these complex behaviors and how to effectively intervene in diverse settings and populations.This issue of