Spacetime supersymmetry and duality in string theory

I discuss the role of spacetime supersymmetry in the interplay between strong/weak coupling duality and target space duality in string theory which arises in string/string duality. This can be seen via the construction of string soliton solutions which in N=4 compactifications of heterotic string theory break more than 1/2 of the spacetime supersymmetries but whose analogs in N=2 and N=1 compactifications break precisely 1/2 of the spacetime supersymmetries. As a result, these solutions may be interpreted as stable solitons in the latter two cases, and correspond to Bogomol'nyi-saturated states in their respective spectra.