String theory, supersymmetry, unification, and all that

String theory and supersymmetry are theoretical ideas that go beyond the st andard model of particle physics and show promise for unifying all forces. After a brief introduction to supersymmetry, the authors discuss the prospe cts for its experimental discovery in the near future. They then show how t he magic of supersymmetry allows us to solve certain quantum field theories exactly, thus leading to new insights about field theory dynamics related to electric-magnetic duality. The discussion of superstring theory starts w ith its perturbation expansion, which exhibits new features including "stri ngy geometry." The authors then turn to more recent nonperturbative develop ments. Using new dualities, all known superstring theories are unified, and their strong-coupling behavior is clarified. A central ingredient is the e xistence of extended objects called branes. [S0034-6861(99)01402-6].