D-effects in toroidally compactified type II string theory

We review exact results obtained for R-4 couplings in maximally supersymmet ric type II string theories. These couplings offer a privileged scene to un derstand the rules of semiclassical calculus in string theory. Upon expansi on 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 cycle s of the compactification manifolds. They also shed light on the relation b etween 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 less tha n or equal to 6 in order to account for the e(-1/g2) effects, still mysteri ous from the point of view of instanton calculus.