STRING DUALITY

The strong-coupling limit of a quantum system is in general quite comp licated, but in some cases a great simplification occurs: the strongly coupled limit is equivalent to the weakly coupled limit of some other system. In string theory conjectures of this type go back several yea rs, but only in the past year and a half has it been understood to be a general principle applying to all string theories. This has improved our understanding of string dynamics, including quantum gravity, in m any new and sometimes surprising ways. The author describes these deve lopments and puts them in the context of the search far the unified th eory of particle physics and gravity.