Mitochondria are widely accepted as an important organelle responsible for numerous processes ranging from ATP production, fatty acid oxidation, as well as the control of cellular apoptosis and steroidogenesis. The mitochondrial genome (mtDNA) exists exclusively within the mitochondrion and in multiple copies within the mitochondrial matrix. In tumorigenesis, modifications are known to arise during the transcription and replication of mtDNA, which often lead to changes in mtDNA copy number. Moreover, different cancers tend to be associated with base changes at various locations within the mitochondrial genome. This chapter discusses current knowledge of the relationship between mtDNA and cancer. The influence of various tumors on mtDNA copy number, and key mtDNA variants associated with disease and cancer will also be discussed, together with the role of mitochondria in tumor cell energy metabolism. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.