The heart has a very high energy demand but very little energy reserves. In order to sustain contractile function, the heart has to continually produce a large amount of ATP. The heart utilizes free fatty acids mainly and carbohydrates to some extent as substrates for making energy and any change in this energy supply can seriously compromise cardiac function. It has emerged that alterations in cardiac energy metabolism are a major contributor to the development of a number of different forms of heart disease. It is also now known that optimizing energy metabolism in the heart is a viable and important approach to treating various forms of heart disease.