An overview is given of the way in which the unification program of particle
physics has evolved into the proposal of superstring theory as a prime
candidate for unifying quantum gravity with the other forces and particles of
nature. A key concern with quantum gravity has been the problem of ultraviolet
divergences, which is naturally solved in string theory by replacing particles
with spatially extended states as the fundamental excitations. String theory
turns out, however, to contain many more extended-object states than just
strings. Combining all this into an integrated picture, called M-theory,
requires recognition of the r\^ole played by a web of nonperturbative duality
symmetries suggested by the nonlinear structures of the field-theoretic
supergravity limits of string theory.