Mitochondria are dynamic intracellular organelles that play a central role in oxidative metabolism and apoptosis. The recent resurgence of interest in the study of mitochondria has been fuelled in large part by the recognition that genetic and/or metabolic alterations in this organelle are causative or contributing factors in a variety of human diseases including cancer. Several distinct differences between the mitochondria of normal cells and cancer cells have already been observed at the genetic, molecular and biochemical levels. As reviewed in this article, certain of these alterations in mitochondrial structure and function might prove clinically useful either as markers for the early detection of cancer or as unique molecular sites against which novel and selective chemotherapeutic agents might be targeted.