The principles of weight management in overweight and obesity are primarily to achieve clinically significant weight loss and secondarily (but perhaps more importantly) to maintain weight loss. The traditional approach to weight reduction has been the prescription of diets that provide an energy intake below that of energy expenditure, usually to provide a balanced ratio of protein, carbohydrate, and fat in reduced quantities to provide an energy intake of 800 to 1500 kcal/d. Evaluation of the efficacy of such interventions is difficult despite a wealth of publications, because few randomized trials have been conducted and various adjunctive therapies confound interpretation. Systematic reviews of low-calorie diets have been unable to come to any firm conclusions about the value of different ways of achieving energy restriction, but one meta-analysis of low-fat diets produced a mean weight loss of 10 kg of weight loss that was greater in those with a higher initial body weight. Greater weight loss was achieved by subjects prescribed a 600-kcal deficit diet, compared with a conventional low-calorie (1200 kcal/d) diet. This study has been influential for clinical dietetic practice, particularly with UK dietitians, but also in the design of dietetic support in clinical trials of pharmacotherapy. Low-calorie diets can be effective treatment, but the optimum way of delivering such diets remains unclear.