The papers in this volume were presented at the 20th Annual International Symposium on Algorithms and Computation, held December 16–18, 2009, in Hawaii, USA. In response to the Call-for-Papers, 279 papers were submitted. Each paper received at least three reviews by either Program Committee members or experts selected by Program Committee members. In all, 120 papers were selected based on the review reports and are included in this volume. We wish to thank all who submitted papers for consideration and all Program Committee members and reviewers for their excellent and hard work. To our invited speakers, we wish to thank you for sharing your expertise. Finally, we wish to thank our colleagues who contributed to the success of the symposium and the sponsors for their assistance and support.

This book constitutes the refereed proceedings of the 20th International Symposium on Algorithms and Computation, ISAAC 2009, held in Honolulu, Hawaii, USA in December 2009. The 120 revised full papers presented were carefully reviewed and selected from 279 submissions for inclusion in the book. This volume contains topics such as algorithms and data structures, approximation algorithms, combinatorial optimization, computational biology, computational complexity, computational geometry, cryptography, experimental algorithm methodologies, graph drawing and graph algorithms, internet algorithms, online algorithms, parallel and distributed algorithms, quantum computing and randomized algorithms.