  We review exact results obtained for R^4 couplings in maximally
supersymmetric type II string theories. These couplings offer a privileged
scene to understand the rules of semiclassical calculus in string theory. Upon
expansion in weak string coupling, they reveal an infinite sum of
non-perturbative e^{-1/g} effects that can be imputed to euclidean D-branes
wrapped on cycles of the compactification manifolds. They also shed light on
the relation between Dp-branes and D-(p-2)branes, D-strings and (p,q) strings,
instanton sums and soliton loops. The latter interpretation takes over in D<=6
in order to account for the e^{-1/g^2} effects, still mysterious from the point
of view of instanton calculus.
  [To appear in the proceedings of the conference "Quantum Aspects of Gauge
Theories, Supersymmetry and Unification" held at Neuchatel University,
Switzerland, 18-23 September 1997.]
