  We review aspects of N=2 duality between the heterotic and the type IIA
string. After a description of string duality intended for the non-specialist
the computation of the heterotic prepotential and the $F_1$ function for the
ST, STU and STUV model (V a Wilson line) and the matching with the Calabi-Yau
instanton expansions are given in detail. Relations with BPS spectral sums in
various connections are pointed out.
