Novel Perspectives in String Phenomenology
I will review the structure of the fermionic Z2xZ2 orbifolds that produced a large space of phenomenological three generation models. I will discuss the trial and error old school method that gave rise to the first known string derived MSSM models, as well as the more modern classification methods that enables the analysis of large spaces of vacua and extraction of some of the global properties underlying them, e.g. spinor--vector duality. I will discuss a new direction of research that was initiated over the past year. Since the mid-eighties it is known that in addition to the ten dimensional tachyon free heterotic-string vacua there exist tachyonic ten dimensional vacua. In the supersymmetric and non-supersymmetric tachyon free cases the ten dimensional tachyon are projected out by the same projection. I will discuss the construction of phenomenological tachyon free models starting from the tachyonic 10D vacua and their particular features.