  The heterotic string theory, compactified to four dimensions, has been
conjectured to have a duality symmetry (S duality) that transforms the dilaton
nonlinearly. If valid, this symmetry could provide an important means of
obtaining information about nonperturbative features of the theory. Even though
it is inherently nonperturbative, S duality exhibits many similarities with the
well-established target-space duality symmetry (T duality), which does act
perturbatively. These similarities are manifest in a new version of the
low-energy effective field theory and in the soliton spectrum obtained by
saturating the Bogomol'nyi bound. Curiously, there is evidence that the roles
of the S and T dualities are interchanged in passing to a five-brane
formulation.
