The chapter mainly deals with the study of energy changes during biochemical reactions which take place inside the body. The reactions are of exergonic (energy releasing) and endergonic (energy consumption). Free energy is the energy available to work. Decrease in the free energy leads to spontaneous reactions to occur. During a chemical reaction, heat may be released or absorbed. The relation between the changes of free energy (DG), enthalpy (DH) and entropy (DS) is expressed as ATP on hydrolysis release-7.3 kcal/mol energy. Substances that release energy higher than ATP, including ATP, are called high energy compounds and they are classified into five groups.