Obesity and its associated health outcomes is one of the major health issues in the United States and in many industrialized nations. Despite the proliferation of weight loss programs and interventions, there are a host of unanswered questions. What constitutes successful weight loss? Why is it so difficult to lose weight? What are the psychosocial processes related to weight loss? What are the best practices in weight loss and weight management behaviors? Are there groups that are disproportionately affected by obesity? This chapter briefly reviews the existing health psychology literature to address some of these questions. It categorizes goals within the weight loss literature, including research on weight loss versus studies on weight maintenance. It also reviews some of the existing debates in weight loss research, such as whether diets should be promoted or if they do more harm than good. This chapter selectively reviews several important psychological theories and models that underlie many weight loss approaches and describes interventions and approaches based on these theories. Last, current issues and future directions for researchers to consider are discussed, including obesity and weight loss among youth and disparity groups. The purpose of this chapter is to highlight the contributions of health psychology in understanding obesity and preventing and intervening on health consequences related to obesity.