This book is about High Intensity Training (HIT) and its scientific background for building lean muscle mass and reducing body fat with two or three workout per week that last only 30 to 50 minutes each. There is a lot of confusion about how to train properly with an incredible variety of suggestions that often contradict eacht other. These questions can be solved by looking at the main questions: a) How does training work? and b) What makes our body adapt in the way we want it do? Sports science has the answers to these questions. High Intensity Training is the logical consequence of the answers that sport science provides us with.