In the strong coupling limit Type IIA superstring theory develops an 11th d
imension that is not apparent in perturbation theory. This suggests the exi
stence of a consistent 11D quantum theory, called M theory, which is approx
imated by 11D supergravity at low energies. In this review we describe some
of the evidence for this picture and some of its implications. (C) 1999 El
sevier Science B.V. All rights reserved.