  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.
