Since mutations in mitochondrial DNA (mtDNA) have been shown to be a cause of many mitochondrial diseases as well as aging, it is important to understand the origin of these mutations and how replication proteins modulate this process. DNA polymerase gamma (pol gamma) is the polymerase that is responsible for replication and repair of mtDNA. Pol gamma has three main roles in mtDNA maintenance and mutagenesis. As the only known DNA polymerase in mitochondria, pol gamma is required for all replication and repair functions and is the main source of errors produced in human mtDNA. Pol gamma is also sensitive to a host of antiviral nucleoside analogs used to treat HIV-1 infections, which can cause an induced mitochondrial toxicity. Finally, the gene for pol gamma, POLG, is a genetic locus for several mitochondrial disease with over 150 genetic mutations currently identified.