  Much curent theoretical analysis is based on the hypothesis that the physics
beyond the Standard Model is a consequence of new principles that occur at the
Planck scale. The question arises whether such principles can ever be directly
tested. We show here that for a significant class of models, hypotheses made at
the string or Planck scale can indeed be directly tested to a relatively high
degree of precision by linear colliders. Three classes of models are examined:
those with universal SUSY soft breaking at the string scale, those with a
horizontal symmetry at the string or Planck scale, and simple Calabi-Yau
superstring models with dilaton and moduli SUSY breaking. (Invited talk at
Orbis97.)

High Energy Physics - Phenomenology