  We discuss higher derivative interactions in the type IIB superstring in ten
dimensions. From the fundamental string point of view, the non-perturbative
corrections are due to D-instantons. We argue that they can alternatively be
understood as arising from $(p,q)$-strings. We derive a non-renormalization
theorem for eight-derivative bosonic interactions, which states that terms
involving either NS-NS or R-R fields occur at tree-level and one-loop only. By
using the $SL(2, Z)$ symmetry of M-theory on $T^2$, we show that in order for
the possible $R^{3m+1} (m=1,2,...)$ interactions in M-theory to have a
consistent perturbative expansion in nine dimensions, $m$ must be odd. Thus,
only $R^{6N+4} (N=0,1,...)$ terms can be present in M-theory and their string
theory counterparts arise at $N$ and $2N+1$ loops. Finally, we treat an example
of fermionic term.
