  We examine the question of the supersymmetric completion of the $R^4$ term in
type IIB supergravity by using superfield methods. We show that while there is
an obstruction to constructing the full action, a subset of the terms in the
action can be consistently analyzed independent of the other terms, and these
can be obtained from the superfield. We find the complete type IIB action
involving the curvature and five-form field strengths.
