Simply put, it's what you're made of: the bones, blood, tissues, fat, water, and everything else that together comprise your body. Weight alone is not a clear indicator of good health because it does not distinguish between pounds that come from body fat and those that come from lean body mass or muscle Why does body composition matter? Knowing your body composition allows you to accurately assess your health and wellness levels, and tells you precisely how your body changes in response to what you do. Body composition enables you to pinpoint changes in your body down to specific quantities of fat loss and muscle gain, helping you set meaningful goals and measure meaningful progress. Let's say you've lost weight. If it's fat, congratulations! But if it's muscle that you've lost, you've probably decreased your strength and lowered the amount of calories you can intake on a daily basis - probably not what you're trying to achieve. However, you would never really know how much of your weight loss is fat versus muscle without measurement of your body composition. Body composition is a more accurate indicator of health than BMI.