According to the American College of Sports Medicine, the mode of exercise you choose should employ large muscle groups in activities that are rhythmic or dynamic in nature. Also, the greatest improvement in cardiorespiratory fitness occurs when exercise involves the use of these large muscle groups over prolonged periods in activities that are also aerobic in nature (running, walking, hiking, swimming, elliptical activity, cycling, etc..) I am giving you these alternative ways so that you don't feel that running is the only way to lose weight. In fact, the risk of injury associated with running is increased if you have just recently started running or if you are overweight due to the stress on your joints and muscles. However, if you feel comfortable running, then it truly is a beneficial mode of exercise for both weight and fat loss. Running involves primarily lower extremity muscles. The quadriceps, hamstrings, hip flexors and extensors, lower leg muscles, and foot muscles are the major groups involved. However, depending on the amount of arm swing, your upper body can also be involved in the motion. Nonetheless, tests have indeed shown that running can help to decrease body weight and fat stores. I also encourage you to perform a resistance training routine to add to your fat loss success. Good Luck!