Deducing L_{fermion} at M_{GUT} using Low Energy Data or Towards a Theory of Fermion Masses

In the last 20 years we have accumulated an enormous amount of data on elementary particles and their interactions. This data serves two purposes: to fix the phenomenological parameters of the Standard Model [SM] and to verify that the SM is an excellent description of nature. It is our goal to understand the origin of these many arbitrary parameters. In this talk we consider a supersymmetric [SUSY] SO(10) grand unified theory [GUT]. We present a straightforward procedure, incorporating a general operator analysis, which allows us to use low energy data to determine the fermionic sector of the theory at the GUT scale