Superstrings beyond one loop

This Laurea Thesis contains six introductory chapters (I-VI) on various aspects of String Theory, mostly related to String compactifications, orientifold constructions and SUSY breaking. On the other hand, the last chapter contains some new results on amplitudes on surfaces with Euler character -1 or -2. These are based on the construction of D’Hoker and Phong, and contain some extensions of their results to type-0 theories.