Target space duality and discrete symmetries in string theory are reviewed in different settings. 1

We study string propagation on AdSs times a compact space from an "old fashioned" worldsheet point of view of perturbative string theory. We derive the spacetime CFT and its Virasoro and current algebras, thus establishing the conjectured AdS/CFT correspondence for this case in the full string theory. Our results have implications for the extreme IR limit of the Dl -D5 system, as well as to 2+1 dimensional BTZ black holes and their Bekenstein-Hawking entropy.

We review some aspects of the interplay between the dynamics of branes in string theory and the classical and quantum physics of gauge theories with different numbers of supersymmetries in various dimensions.

A double scaling limit can be defined in string theory on a Calabi-Yau (CY) manifold by approaching a point in moduli space where the CY space develops an isolated singularity and at the same time taking the string coupling to zero, while keeping a particular combination of the two parameters fixed. This leads to a decoupled theory without gravity which has a weak coupling expansion, and can be studied using a holographically dual non-critical superstring description. The usual "Little String Theory" corresponds to the strong coupling limit of this theory. We use holography to compute two and three point functions in weakly coupled double scaled little string theory, and study the spectrum of the theory in various dimensions. We find a discrete spectrum of masses which exhibits Hagedorn growth. 9/99