We define the notion of exact completion with respect to an existential elementary doctrine. We observe that the forgetful functor from the 2category of exact categories to existential elementary doctrines has a left biadjoint that can be obtained as a composite of two others. Finally, we conclude how this notion encompasses both that of the exact completion of a regular category as well as that of the exact completion of a category with binary products, a weak terminal object and weak pullbacks. MSC 2000: 03G30 03B15 18C50 03B20 03F55