It is known that every complex of leftR-modules has an exact cover. We consider the question of findingRfor which every complex of left i?-modules has an exact envelope. We first give a simple proof that for everyRevery complex of leftR-modules has a special exact preenvelope and then use this to show that the left perfect rings are thoseRfor which every complex has an exact envelope. So, for example, this result holds for all artinian rings.