This collection of reviews and protocols provides the reader with an introduction to the current state of knowledge on how various diseases are related to mitochondrial dysfunction. Mitochondria contain their own genome, a small, circular double-stranded DNA (mtDNA), and alterations in mtDNA may play an important role in the multistep carcinogenesis of at least some types of human cancer. In addition to mutations of mtDNA, many mitochondrial syndromes are due to abnormalities in nuclear genes related to oxidative phosphorylation (OXPHOS). Mitochondrial tRNA (MTT) gene mutations are an important cause of human morbidity and are associated with a wide range of pathology, from isolated organ?]specific diseases such as myopathy or hearing loss, to multisystem disorders with encephalopathy, gastrointestinal dysmotility, and life?]threatening cardiomyopathy. The relationship of the mitochondrion organelle to aging and longevity is also discussed.Laboratory protocols describe methodology to characterize mtDNA heteroplasmy by parallel sequencing. Each eukaryotic cell contains hundreds of mitochondria with hundreds of mitochondria genomes. Mutant and wild-type mtDNA may co-exist as heteroplasmy, and cause human disease. The purpose of this protocol is to simultaneously determine mtDNA sequence and quantify the heteroplasmic level. Another protocol describes procedures for obtaining tissue sections and cell material suitable for histological evaluation of OXPHOS activity and integrity and immunodetection of the complexes in tissue from patients suspected of mitochondrial disease. Emphasis lies on the diagnostic potential of these techniques to differentiate mtDNA from nuclear mutations.This e-book — a curated collection from eLS, WIREs, and Current Protocols — offers a fantastic introduction to the field of mitochondrial diseases for students or interdisciplinary collaborators.Table of Contents:IntroductionMitochondrial DNA Copy Number Alterations in Human Cancers eLSMan YuMitochondrial Disorders: Nuclear Gene Mutations eLSDaniele Ghezzi and Massimo ZevianiMitochondria as a Key Determinant of Aging eLSLudivine Walter and Siu Sylvia LeeAdvanced ReviewsMitochondrial tRNA Mutations and Disease WIREs RNAJohn W. Yarham, Joanna L. Elson, Emma L. Blakely, Robert McFarland, Robert W. TaylorHuman Mitochondrial Diseases Caused by Lack of Taurine Modification in Mitochondrial tRNAs WIREs RNASuzuki Tsutomu, Nagao Asuteka, Suzuki TakeoProtocolsNext Generation Sequencing to Characterize Mitochondrial Genomic DNA Heteroplasmy Current Protocols in Human GeneticsTaosheng HuangHistochemical Methods for the Diagnosis of Mitochondrial Diseases Current Protocols in Human GeneticsBoel De Paepe, Jan L. De Bleecker, Rudy Van Coster