The prevalence of obesity has risen markedly in California over the past decade, paralleling nationwide trends. The costs and consequences associated with excess weight have led some researchers to argue that obesity now constitutes a greater public health threat than other leading health risks, such as tobacco use and excessive alcohol consumption. This study offers a detailed examination of obesity in California, paying particular attention to racial and ethnic differences. In addition, it explores how both individual characteristics and neighborhood environments relate to obesity risk. Finally, it considers the effectiveness of a variety of obesity prevention efforts and interventions.