The chapter begins with Zoom In \"Utility of human bioenergetics\". It is emphasized that chemical energy trapped within the bonds of carbohydrates, proteins and fats cannot be used directly by the body unless it is converted into ‘usable form’. This usable form is ATP. (Adenosine Triphosphate). In another Zoom In \"A miracle called ATP\" various functions of ATP, i.e. mechanical work, Pumping action, Increase in temperature, Decrease in temperature, Powering electronic circuits are aptly described. ATP can be compared only with electricity in its versatility. This follows discussion on ‘Fuel’ for the muscle contraction wherein ATP–Creatine phosphate (CP) system. Glycogen–Lactic acid system (Glycolytic system) and Aerobic system are elaborated. Sports that utilize different types of energy systems are enlisted. The concept of Lactate threshold is explained with the help of a diagram. WHY BOX deals with \"Why blood lactate threshold is different in ‘aerobic’ and ‘anaerobic’ athletes?\" A brief write up on Modern theory to explain Oxygen Debt now known as \'recovery oxygen uptake\' follows. Finally a common Myth viz. \"Lactate is a waste product of muscle metabolism.\" is dispelled.