Intentional weight loss improves many of the medical complications associated with obesity. Moreover, many of these beneficial effects have a dose-dependent relationship with the amount of weight lost and begin after only modest weight losses of 5% to 10% of initial body weight. There is no conclusive evidence that weight loss decreases mortality in obese people. The therapeutic effect of weight loss on risk factors for cardiovascular disease (insulin resistance and diabetes, dyslipidemia, and hypertension) has received the most attention in clinical trials. The hazard of developing coronary heart disease is directly related to the concomitant burden of risk factors. Modest weight loss can affect the entire cluster of risk factors simultaneously. Both negative energy balance and weight loss improve insulin sensitivity and glycemic control in obese patients with type 2 diabetes. Most studies have found that weight loss decreases serum triglyceride, total cholesterol, and low-density lipoprotein cholesterol concentrations and increases serum high-density lipoprotein cholesterol concentration. Regain of weight leads to relapse in triglyceride and cholesterol concentrations. Weight loss, independent of sodium restriction, decreases systolic and diastolic blood pressure. Dietary intervention is the cornerstone of weight-loss therapy. Most diets proposed for losing weight vary in two principal dimensions: energy content and macronutrient composition. Manipulation of food macronutrient content, energy density, and portion size can help decrease energy intake and facilitate weight loss.