This book constitutes the thoroughly refereed post workshop proceedings of the 5th International Workshop on Approximation and Online Algorithms, WAOA 2007, held in Eilat, Israel, in October 2007 as part of the ALGO 2007 conference event. The 22 revised full papers presented were carefully reviewed and selected from 56 submissions. The workshop covered areas such as algorithmic game theory, approximation classes, coloring and partitioning, competitive analysis, computational finance, cuts and connectivity, geometric problems, inapproximability results, mechanism design, network design,...