It depends on what kind of fat. Saturated fat in high quantities won't make you fat, but it will impair heart health, similarly to "bad" cholesterol, that said, but it IS necessary as it transports calcium to the bones. Monounsaturated fats help counteract the negative effects of saturated fat as well as help lower cholesterol, so it's best to strike a good balance between the two, polyunsaturated fats do this as well, but they also lower good cholesterol in the process. Trans fats are just bad, avoid them like the plague, they will collect right alongside natural fat, but won't go away as easily, additionally they become what's known as plague in the blood stream, which increases the likelihood of clotting and gets in the way of oxygenation, resulting in chronic shortness of breath and muscle fatigue. Anything containing hydrogenated or partially hydrogenated oil will be converted to trans fats inside the body, and will thus have the same effects. Fat doesn't make you fat, carbohydrates make you fat.