Over the past decade string theory has had an increasing impact on many areas of physics: high energy and hadronic physics, gravitation and cosmology, mathematical physics and even condensed matter physics. The impact has been through many major conceptual and methodological developments in quantum field theory in the past fifteen years. In addition, string theory has exerted a dramatic influence on developments in contemporary mathematics, including Gromov - Witten theory, mirror symmetry in complex and symplectic geometry, and important ramifications in enumerative geometry. This volume is derived from a conference of younger leading practitioners around the common theme: 'What is string theory?' The talks covered major current topics, both mathematical and physical, related to string theory.