In eukaryotic cells, mitochondria are the primary source of ATP, which is generated by oxidative phosphorylation. Mammalian and human mitochondria contain their own genome, which is maternally inherited. Defects in mitochondrial gene expression result in a number of human diseases and contribute to aging. Transcription of mitochondrial genes is carried out by unique transcription machinery, consisting of a single-subunit bacteriophage T7-like mitochondrial RNA polymerase and several nuclear-encoded transcription factors. Mitochondrial transcription (and, consequently, oxidative phosphorylation) may be regulated by transcription initiation and termination factors and changes in ATP levels in response to alterations in cell metabolic demands. Recent data suggest that mitochondrial transcription is coordinated with other crucial processes, such as DNA replication and translation, indicating the importance of studies of the molecular mechanisms of mitochondrial gene expression.