Blood Doping: What It Is and Why Lance Didn't Get Caught
Blood doping is done through transfusions or through EPO injections.
Athletes who do it try to increase their number of red blood cells, and ultimately their oxygen-carrying capacity so they can last longer and go faster.
Blood doping can be hard to detect because there aren't the physical symptoms that you see with other performance-enhancing drugs like steroids (common side effects are acne and aggressive behavior).
Since 2009 high-profile races the The Tour De France have tested the blood levels in athletes.