The study tests the effectiveness for weight loss and weight maintenance of four diets differing in macronutrient composition: moderate in fat (40% energy) with two different protein levels (15% and 25%), and low in fat (20% energy), also with 15% and 25% protein levels. The moderate-fat diet will be patterned after a Mediterranean diet. All four dietary approaches will be low in saturated fat, and will involve reduction in total energy intake. Each diet is deemed practical and suitable for public health recommendations, and each would be expected to have a favorable effect on cardiovascular disease risk factors. All participants will receive a state-of-the-art behavioral therapy program standardized across the two centers in Boston and Baton Rouge. An estimated 800 men and women, age 30-70 years, body mass index (BMI) 25-40 kg/m2, will be randomized among the 4 dietary treatments. The primary outcome variable will be change in total body weight from baseline to 2 years. Secondary outcomes related to obesity are body fat, BMI, waist circumference, visceral fat, and hepatic and skeletal muscle fat. Other outcomes are psychological factors (diet satisfaction, satiety, food craving, dietary restraint, disinhibition and hunger, and quality of life); major cardiovascular risk factors (blood pressure, low density lipoprotein [LDL] cholesterol, high density lipoprotein [HDL] cholesterol, and triglycerides); prevalence of the metabolic syndrome; blood glucose, insulin, and hemoglobin A1C; emerging cardiovascular risk factors (apolipoprotein B, VLDL and LDL particles with apolipoprotein C-III, lipoprotein[a]); microalbuminuria; and bone mineral content. The primary results will be straightforwardly applicable to public health and clinical guidelines for obesity and will increase our understanding of the biology of obesity and weight loss.