  We give a complete characterization of the continuity of pseudodifferential
operators with symbols in modulation spaces $M^{p,q}$, acting on a given
Lebesgue space $L^r$. Namely, we find the full range of triples $(p,q,r)$, for
which such a boundedness occurs. More generally, we completely characterize the
same problem for operators acting on Wiener amalgam space $W(L^r,L^s)$ and even
on modulation spaces $M^{r,s}$. Finally the action of pseudodifferential
operators with symbols in $W(\Fur L^1,L^\infty)$ is also investigated.
