Better sex. Increased vitality. More muscle. Improved health. Greater mental agility. These are just a few of the life-enhancing benefits that men with low levels of testosterone can experience when they increase their testosterone level. If you’ve noticed a decrease in your sex drive; experienced erectile dysfunction; or felt tired, depressed, and unmotivated, this authoritative, up-to-date guide from an expert at Harvard Medical School will help you determine if you have low testosterone–a surprisingly common but frequently undiagnosed condition among middle-aged men.