  We develop techniques to compute higher loop string amplitudes for twisted
$N=2$ theories with $\hat c=3$ (i.e. the critical case). An important
ingredient is the discovery of an anomaly at every genus in decoupling of BRST
trivial states, captured to all orders by a master anomaly equation. In a
particular realization of the $N=2$ theories, the resulting string field theory
is equivalent to a topological theory in six dimensions, the Kodaira--Spencer
theory, which may be viewed as the closed string analog of the Chern--Simon
theory. Using the mirror map this leads to computation of the `number' of
holomorphic curves of higher genus curves in Calabi--Yau manifolds. It is shown
that topological amplitudes can also be reinterpreted as computing corrections
to superpotential terms appearing in the effective 4d theory resulting from
compactification of standard 10d superstrings on the corresponding $N=2$
theory. Relations with $c=1$ strings are also pointed out.
