This book is based on the author's PhD thesis which was selected as the winning thesis of the 1993 ACM Doctoral Dissertation Competition. The author improved the presentation and included the progress achieved since the thesis was approved by the University of California at Berkeley.This work is a fascinating piece of theoretical computer science research building on deep results from different areas. It provides new theoretical insights and advances applicable techniques in such different areas as computational complexity, efficient (randomized) checking of proofs, programs and polynomials, approximation algorithms, NP-complete optimization, and error-detection and error-correction algorithms in coding theory.