Classification of the Chiral Z_2 x Z_2 Heterotic String Models
  This thesis provides a classification of the chiral content of the heterotic $\mathbbm{Z}_2 \times \mathbbm{Z}_2$ orbifold models. We show that the chiral content of the heterotic $\mathbbm{Z}_2 \times \mathbbm{Z}_2$ orbifold models at any point in the moduli space can be described by a free fermionic model. We present a direct translation between the orbifold formulation and the free fermionic construction. We use the free fermionic description for the classification wherein we consider orbifolds with symmetric shifts.   We show that perturbative three generation models are not obtained in the case of $\mathbbm{Z}_2 \times \mathbbm{Z}_2$ orbifolds with symmetric shifts on complex tori, and that the perturbative three generation models in this class necessarily employ an asymmetric shift. We show that the freedom in the modular invariant phases in the $N = 1$ vacua that control the chiral content, can be interpreted as vacuum expectation values of background fields of the underlying $N = 4$ theory, whose dynamical components are projected out by the $\mathbbm{Z}_2$ fermionic projections. In this class of vacua the chiral content of the models is determined by the underlying $N = 4$ mother theory. 
S.E.M. Nooij