  We review the status of solitons in superstring theory, with a view to
understanding the strong coupling regime. These {\it solitonic} solutions are
non-singular field configurations which solve the empty-space low-energy field
equations (generalized, whenever possible, to all orders in $\alpha'$), carry a
non-vanishing topological "magnetic" charge and are stabilized by a topological
conservation law. They are compared and contrasted with the {\it elementary}
solutions which are singular solutions of the field equations with a
$\sigma$-model source term and carry a non-vanishing Noether "electric" charge.
In both cases, the solutions of most interest are those which preserve half the
spacetime supersymmetries and saturate a Bogomol'nyi bound. They typically
arise as the extreme mass=charge limit of more general two-parameter solutions
with event horizons. We also describe the theory {\it dual} to the fundamental
string for which the roles of elementary and soliton solutions are
interchanged. In ten spacetime dimensions, this dual theory is a superfivebrane
and this gives rise to a string/fivebrane duality conjecture according to which
the fivebrane may be regarded as fundamental in its own right, with the
strongly coupled string corresponding to the weakly coupled fivebrane and
vice-versa. After compactification to four spacetime dimensions, the fivebrane
appears as a magnetic monopole or a dual string according as it wraps around
five or four of the compactified dimensions. This gives rise to a
four-dimensional string/string duality conjecture which subsumes a
Montonen-Olive type duality in that the magnetic monopoles of the fundamental
string correspond to the electric winding states of the dual string. This leads
to a {\it duality of dualities} whereby under string/string duality the the
strong/weak coupling $S$-duality trades places with the minimum/maximum length
$T$-duality. Since these magnetic monopoles are extreme black holes, a
prediction of $S$-duality is that the corresponding electric massive states of
the fundamental string are also extreme black holes.
