Myocardial tissue is highly dependent on energy supplied by normal mitochondrial function. Therefore defects of energy production or utilization affect the heart in both syndromic and isolated disorders. Knowledge of the peculiar structural, functional, and genetic characteristics of mitochondria provides the basis for identification and classification of mitochondrial defects as well as for establishment of a diagnostic workup useful for related cardiac disorders. This review is therefore dedicated to the characteristics of normal mitochondria and the pathologic alterations of these organelles in various cardiovascular diseases.