There is no doubt that mitochondria perform a very important role in eukaryotic cells. Mitochondria are the actual respiratory organs of the cells, where the food stuff (i.e. carbohydrates and fats) are completely oxidized into Co2 and water. During the biological oxidation of carbohydrates and fats, large amount of energy is released. This energy is utilized by mitochondria for synthesis of the energy rich compounds, known as -Adenosine tri-phosphate-ATP, so mitochondria are called "Power Houses "of the cell. This made scientists be more attention to mitochondrial role in eukaryotic cells and study their structure and their real origin. So, many hypothesizes were established to reach the mitochondrial origin and many techniques were developed and used for discovering the origin and function of each component found in them. Therefore, in this book I tried to exposure this important subject with an easy word, including valuable body of knowledge covering all the aspects of this subject to reach with you the most beneficial information.