The morbidity and mortality of coronary heart disease and of heart failure remain unacceptably high despite major advances in their management. The main focus of treatment has been revascularisation for ischaemic heart disease and neuro-humoral modification for heart failure. There is an urgent need for new modalities of treatment to improve mortality and morbidity. Recently, there has been a great deal of interest in the role of disturbances in cardiac energetics and myocardial metabolism in the pathophysiology of both ischaemic heart disease and heart failure and of therapeutic potential of metabolic modulation. The myocardium is a metabolic omnivore, but mainly uses fatty acids and glucose for generation of Adenosine-5-triphosphate (ATP). This review focuses on the key changes that occur to the metabolism of the heart in ischaemia and in heart failure and its effects on cardiac energetics.