The past twenty years have witnessed the accumulation of a large body of knowledge on the formation of mitochondria. We do not intend to cover this information in detail; instead, we review the main facts that have been learn, ed since the advent of recombinant DNA technologies, discuss the new principles that have emerged, and outline some of the questions that are still unanswered. Our discussion focuses on the genes contained in mitochondrial DNA (mtDNA), how these genes are organized and exн pressed, and how the nuclear genome contributes to mitochondrial bioн genesis. A more detailed treatment of some of these topics, as well as a surн vey of earlier work, can be found in several excellent reviews (Gillham 1978; Tzagoloff & Myers 1986; Cantatore & Saccone 1987).