This volume contains the proceedings of the Workshop on On-line Algorithms held at the DIMACS Center at Rutgers University in February 1991. Presenting results in the theory of on-line algorithms, the articles discuss a broad range of problems. Most of the papers are based on competitive (worst-case) analysis of on-line algorithms, but some papers consider alternative approaches to on-line analysis. A critical question examined by some of the authors is how to modify competitive analysis to better reconcile the theory and practice of on-line algorithms. Many of the papers examine the ways in which randomization can be used to yield algorithms with improved performance. This book is aimed primarily at specialists in algorithm analysis, but most of the articles present clear expositions of previous work.