In recent years we are understanding the relative importance of knowing a person's body composition, not just their weight. The two component model of body density starts with the assumption that your body has two components: fat and fat-free weight. It is fat-weight that seems to be implicated in many diseases. Therefore, we are becoming interested in our body fat levels and in learning how to more accurately measure the body's fat component. The photo in the multimedia slide illustrates underwater weighing also called "hydrostatic weighing." This method has been long considered the "gold standard" for comparing methods of determining body fat. Though hydrostatic weighing is considered a standard, there is still a significant error factor when measuring body fat, no matter what method is used. Body fat measures must be considered along with other measures of health and fitness to form a reasonably accurate basis for exercise and nutrition prescription.