The mitochondrion, long considered an organelle specific to energy metabolism, is in fact multi-functional and involved in many diseases. Mitochondrial DNA accumulates somatic mutations during aging, the progression of cancer and diabetes. Most cancer cells contain homoplasmic mutations in the mitochondrial genome. Although little is known about the contributions of mutations to carcinogenesis, some mutations in the nuclear genes encoding mitochondrial proteins have been identified as responsible for certain familial cancers. Mitochondria play an essential role in generating the germ line by releasing mitochondrial ribosomal RNAs, by which the germ line transfers the genetic information necessary for life to the next generation. Collaboration between mitochondria and the cytosol occurs in several metabolic pathways. Many enzymes involved in synthesizing uridine, heme and steroids and in the urea cycle are located inside mitochondria. Notably, a reaction involved in the synthesis of UMP is coupled with the energized state of mitochondria. Thus, the synthesis of DNA and RNA should be indirectly coupled with the energized state of mitochondria. Additionally, storing calcium is an important role of mitochondria. Calcium functions as a second messenger in signal transduction, however, it also activates several proteinases or lipases to induce damage. The mitochondrion plays a significant role in necrosis and is a center for apoptosis, determining its initiation, regulation and execution. Thus, the mitochondrion is widely involved in cell proliferation, cell death and disease.