What can we learn from the study of non-perturbative quantum general   relativity?
  I attempt to answer the question of the title by giving an annotated list of the major results achieved, over the last six years, in the program to construct quantum general relativity using the Ashtekar variables and the loop representation. A summary of the key open problems is also included. Also included are expositions of several new results including the construction of spatially diffeomorphism invariant observables constructed by coupling general relativity to matter fields. 
Lee Smolin