Strings, Unification and Dilaton/moduli-induced Susy-breaking

I discuss several issues concerning the use of string models as unified theories of all interactions. After a short review of gauge coupling unification in the string context, I discuss possible motivations for the construction of SU(5) and SO(10) String-GUTs. I describe the construction of such String-GUTs using different orbifold techniques and emphasize those properties which could be general. Although SO(10) and SU(5) String-GUTs are relatively easy to build, the spectrum bellow the GUT scale is in general bigger than that of the MSSM and includes colour octets and SU(2) triplets. The phenomenological prospects of these theories are discussed. I then turn to discuss soft SUSYbreaking terms obtained under the assumption of dilaton/moduli dominance in SUSY-breaking string schemes. I underline the unique finiteness properties of the soft terms induced by the dilaton sector. These improved finiteness properties seem to be related to the underlying SU(1, 1) structure of the dilaton couplings. I conclude with an outlook and some speculations regarding N = 1 duality.