  Massless BPS-saturated states in toroidally compactified heterotic string
were discussed first. Then, I constructed a large class of BPS-saturated states
of toroidally compactified type II string theories. They were parametrized by
four independent parameters. Both orthogonal and non-orthogonal intersections
of non-perturbative states in the corresponding ten dimensional theories were
discussed. Phenomenologically, I considered the string effect in the form of
threshold corrections on dilatonic charged black holes, and the superpotential
of N=1 supergravity theory
