  One-loop corrections to the gauge coupling and the gauge kinetic function in
certain classes of four-dimensional D-brane models are computed. It is
described how to determine D-instanton corrections to the superpotential and
the gauge kinetic function in such models. The Affleck-Dine-Seiberg
superpotential is rederived in string theory. The existence of a new class of
multi-instantons, dubbed poly-instantons, is conjectured.
