This book constitutes the thoroughly refereed workshop post-proceedings of the 18th International Workshop on Approximation and Online Algorithms, WAOA 2019, held virtually in  September 2020 as part of ALGO 2020. The 15 revised full papers presented this book were carefully reviewed and selected from 40 submissions. Topics of interest for WAOA 2018 were graph algorithms, inapproximability results, network design, packing and covering, paradigms for the design and analysis of approximation and online algorithms, parameterized complexity, scheduling problems, algorithmic game theory, algorithmic trading, coloring and partitioning, competitive analysis, computational advertising, computational -finance, cuts and connectivity, geometric problems, mechanism design, resource augmentation, real-world applications. Chapter "Explorable Uncertainty in Scheduling with Non-Uniform Testing Times" is available open access under a Creative Commons Attribution 4.0 International License via link.springer.com.