In writing this book, I found the choice of a suitable title to be a most vexing problem. Lehninger's excellent earlier monograph The Mitochondrion had already appropriated in the domain of library cards what appeared to be the most fitting description of the subject matter. Once the text was completed, however, it became obvious that pluralization was the simplest solution to this dilemma. Variations in the structure and function of this organelle and recent discoveries of the phylogenetic diversity in the organization and genetic con­ tent of its DNA all seemed to justify the idea that there are as many different mitochondria as there are mitochondriologists. Even though my initial intention was to provide advanced undergraduate and graduate students of cell biology with supplemental reading material on topics usually dealt with in a cursory manner by most standard texts, inevitably the scope was broadened to attract the interest of more seasoned readers who might be familiar with some but not other areas of mitochondrial studies. Con­ sistent with the original aim, literature citations have been kept to a minimum and are avoided in the main body of the text for purposes of readability.