String Model Building in the Age of D-Branes
  The string duality revolution calls into question virtually all of the working assumptions of string model builders. A number of difficult questions arise. I use fractional charge as an example of a criterion which one would hope is robust beyond the weak coupling heterotic limit. Talk given at the 5th International Workshop on Supersymmetry and Unification of Fundamental Interactions (SUSY-96), University of Maryland, College Park, May 29 - June 1, 1996. 
Joseph D. Lykken