Permutation Orbifolds in Conformal Field Theories and String Theory

This work covers various topics in modern Theoretical Physics, in the attempt of describing our world. In particular, the subject of String Theory and related fields are considered. Two-dimensional Conformal Field Theories play a crucial role in the construction of realistic four-dimensional string models. Results in CFT's have a broader validity since their range of applications goes beyond String Theory. Here new CFT results are given, for a particular class of theories (namely the permutation orbifold), and then used in the context of string model building.