Obesity is an epidemic health crisis that is due, in part, to inadequate exercise. To effectively prescribe walking as exercise for obese adults requires knowledge of metabolic energy expenditure and the risk of musculoskeletal injury during walking. To test the hypothesis that walking would be more metabolically expensive for obese adults, I measured the metabolic rate of obese and normal weight females and males while they walked on a level treadmill at six speeds (0.50-1.75 m/s). Net metabolic rates of obese subjects were only ~10% greater (per kg) than normal weight subjects of the same sex and net metabolic rates for females were ~10% greater than males. Differences in body mass distribution did not explain the differences in net metabolic rate between females and males, rather, overall body composition was the best predictor of net metabolic rate. To determine why obesity has only a modest effect on net metabolic rate, I simulated obesity in normal weight adults by having them walk on a level treadmill while carrying loads on their waist, thighs, shanks or feet. I measured metabolic rate, kinematics and muscle activity and found that net metabolic rate increased with load mass and more distal location. Thigh loads were carried relatively inexpensively, helping to explain the small difference in net metabolic rate between obese females and males. Sagittal plane kinematics, muscle moments and EMG were similar with no-load, thigh and shank loads and thus could not explain the increase in metabolic cost. Frontal plane mechanics may be an important determinant of the metabolic cost of walking. To determine whether obesity increases loads on the lower extremity joints while walking, I measured the ground reaction forces, kinematics and joint torques of obese and normal weight adults while they walking on a level treadmill at six speeds (0.5-1.75 m/s). Obese adults had much greater sagittal and frontal plane knee torques, suggesting greater medial compartment knee joint loads that may increase the risk of osteoarthritis. Slow walking may be an appropriate exercise prescription for obese adults, given that it increases the energy expended per distance walked and reduces joint loads.