Mitochondria are commonly known as the "power house" of cells. These are the distinct organelles of all eucaryotic cells, plants, fungi and some protists. In the cell, mitochondria are the sites of aerobic respiration. They are enclosed by a double membrane. The inner membrane is thrown into folds which are called as 'cristae'. Oxidative enzymes are located on these cristae. Mitochondria contain circular and naked DNA. They reproduce by fission, like bacteria. In the absence of mitochondria, eucaryotic cells would have to respire anaerobically and they will have to depend on an inefficient process, like glycolysis for their energy needs. It is interesting that mitochondria resemble an aerobic and non-phbtosynthetic bacterium like Paracoccus. In fact, in one of the eucarjotes like Pleomp pcllustris, there are no mitochondria. In this organism, in place of mitochondria there are serobic bacteria which help the organism in respiration. The ass~ciationo f bacteria with the organism shows a kind of symbiotic (mutual advantage) relationship, wherein bacteria help organism in respiration and in turn bacteria are benefitted by living at the expense of dK organism permanently. This relationship suggests that mitochondria are procaryotes which have entered the "eucaryotic" cell as endosymbionts, i.e.. they have association inside the cell evenbefore the entry of chloroplasts in the cell, In other words, we can say that mitochondria have evolved earlier than the chloroplasts.