Hydrogenosomes and Mitosomes: Mitochondria of Anaerobic Eukaryotes" provides a summary of the current knowledge of these organelles, which occur in unicellular, often parasitic organisms, including human pathogens. The distribution of these organelles is broad, but they were detected primarily in an anoxic habitat or nutrient rich intracellular niche that permits life without the efficient energy generating system of typical mitochondria. Their common characteristic is that they lack the aerobic energy conservation system of typical mitochondria and that they are usually the site of the synthesis of iron-sulfur clusters, regarded as the only indispensable function of eukaryotic mitochondria.