Parameter Space Investigations of Free Fermionic Heterotic Models

I survey the parameter space of NAHE-based free fermionic heterotic string models. First, I discuss flat directions of the low energy effective field theories and show that D-flat directions need not be isomorphic to gauge invariant superpotential terms. Next, I review recent studies of three generation SU(3)_C x SU(2)_L x U(1)_Y, SU(3)_C x SU(2)_L x SU(2)_R x U(1)_B-L, SU(4)_C x SU(2)_L x U(1)_Y', and flipped SU(5) models, constructed by Wilson breaking of the underlying SO(10) observable sector gauge group of the NAHE Z_2 x Z_2 basis. Based on talk presented at the First International Conference on String Phenomenology.