Mitochondria are the organelles responsible for energy production that 'house' many pathways of intermediary metabolism. It should not be surprising, therefore, that several human diseases involve mitochondrial dysfunction or dysregulation, although many of these diseases have complex etiologies that are not yet fully defined. For some of these diseases, there is evidence that ameliorating the mitochondrial dysfunction will provide clinical benefit. Several marketed or late-stage drugs are now known to act on mitochondrial targets, although this was not recognized when they were initially developed. The main requirements for progress in the area of mitochondrial drug development are a more systematic and comprehensive definition of the mitochondrial proteome and the identification of targets for drug development.