We develop and extend the theory of Mackey functors as an application of enriched category theory. We define Mackey functors on a lextensive category E and investigate the properties of the category of Mackey functors on E . We show that it is a monoidal category and the monoids are Green functors. Mackey functors are seen as providing a setting in which mere numerical equations occurring in the theory of groups can be given a structural foundation. We obtain an explicit description of the objects of the Cauchy completion of a monoidal functor and apply this to examine Morita equivalence of Green functors.