There is a widespread epidemic of obesity in the United States, which has been associated with an increased risk of diabetes mellitus, cancer, and cardiovascular diseases. Although lifestyle modifications and long-term dietary vigilance remain cornerstones of weight reduction treatment, the continued availability of U.S. Food and Drug Administration-approved pharmacotherapies has expanded the options available for the management of obesity. These agents include anorexiants, thermogenic drugs, and lipid-partitioning drugs. As knowledge regarding the possible causes of obesity increases, there are new drugs under investigation, which include beta3-adrenergic receptor agonists, modifiers of leptin, and cannabinoid receptor-1 antagonists (rimonabant). Also under investigation are antidiabetic agents (metformin, exenatide), anticonvulsant drugs (topiramate, zonisamide), antidepressants (bupropion, fluoxetine), and growth hormones. New targets for pharmacotherapy include uncoupling proteins, fatty acid synthase, neuropeptide Y, melanocortin, ghrelin, various regulatory gut peptides, and ciliary neurotropic factor. Pharmacologic agents are in clinical development that target these substances.