Mitochondria provide the cell's energy through respiration. Their DNA, discovered in 1963, display a number of peculiarities which fascinate researchers. Indeed, each cell contains several hundred copies of this small DNA which results from a longstanding process of endosymbiosis. This mechanism enabled the initial mitochondrial DNA to lose information towards the nucleus, enabling higher copy. The high copy number and polymorphism of the mitochondrial DNA allowed to reveal that our common "Eve" lived in Africa about 200,000 years ago, that the Neanderthalian progressively extinguished about 40,000 years ago, and that some remains of bones belonged to tsar Nicholas II and to his family. A novel class of diseases has been discovered in 1988 as resulting from mtDNA alterations, and from maternally inherited mutations. A possible mechanism to account for the origin of large-scale deletions is argued on the basis of the formation of a DNA triple helix during replication. This book, intended to a wide audience and biochemists, describes the main historical characteristics of the discoveries of the mitochondrial DNA, among which the author was involved, notably in mitochondrial diseases.