For more than a century after their discovery, mitochondria were viewed almost exclusively as ATP-generating metabolic units. This view changed with the discovery of the mitochondrial genetic system and the central role of mitochondria in programmed cell death. As a result, much of the current research focuses on the complex interplay between mitochondria and the rest of the eukaryotic cell. This interplay is central to mitochondrial biogenesis. The following five reviews published in this volume summarize recent discoveries in this area: DNA Replication and Transcription in Mammalian Mitochondria, Mitochondrial-Nuclear Communications, Translocation of Proteins into Mitochondria, The Machines that Divide and Fuse Mitochondria, and Why Do We Still Have a Maternally Inherited Mitochondrial DNA? Insights from Evolutionary Medicine.