  A study of zero-dimensional theories, based on exact results, is presented.
First, relying on a simple diagrammatic representation of the theory, equations
involving the generating function of all connected Green's functions are
constructed. Second, exact solutions of these equations are obtained for
several theories. Finally, renormalization is carried out. Based on the
anticipated knowledge of the exact solutionsthe full dependence on the
renormalized coupling constant is studied.
