ABSTRACT To fully understand the basic biology that underlies human aging, and accurately time potential treatments that are aimed at preventing age-related pathology, it will be of vital importance to determine when the events that precipitate human aging occur. One of the processes that drives human aging is mitochondrial mutagenesis. Mitochondrial DNA (mtDNA) mutations accumulate as we grow older, which accelerates the natural aging process and contributes to various age-related diseases, including cancer, muscle wasting and neurodegeneration. However, due to the multiplicity of mitochondrial genomes in a cell, de novo mtDNA mutations are initially harmless. MtDNA mutations need to clonally expand to cause disease. Because this expansion process takes time, we hypothesize that the mutations that precipitate age-related pathology arise relatively early in life, and that the pace of mitochondrial aging is set long before pathology becomes apparent. We propose to test this hypothesis with a new mouse model of DNA polymerase gamma (PolgA), the enzyme that replicates the mitochondrial genome. This model expresses an error prone version of DNA polymerase gamma that can be replaced with a WT allele at will. Accordingly, we can turn off mitochondrial mutagenesis at any time during the animal’s lifespan and determine how mutations that occur early in life affect pathology later in life. In addition, we will use various mutation detection techniques, including random mutation capture, single cell sequencing and duplex sequencing to track the fate of these mutations over the lifespan of our mice. These experiments will describe the natural history of every possible mtDNA mutation in various tissues, effectively dissecting the parameters that control the impact of mtDNA mutations on human health. Finally, we propose to rejuvenate somatic cells and aging mice by manipulating mitochondrial fusion and mitophagy, in an attempt to cure them from deleterious mtDNA mutations. If successful, this strategy could provide a potential treatment for multiple pediatric mtDNA diseases, as well as the mitochondrial component of age-related diseases. Accordingly, our experiments have the potential to revolutionize our understanding of the relationship between mitochondrial mutagenesis and aging, and provide powerful new tools to combat both inherited and age-related diseases that are associated with mtDNA mutations.