The book "Algorithms - ESA 2013" documents the refereed contributions of the 21st Annual European Symposium on Algorithms, which took place in September 2013 in Sophia Antipolis, France. It includes 69 revised full texts selected from 303 original submissions. The contributions cover a wide range of algorithmic research areas, including algorithmic aspects of networks, algorithmic game theory, approximation algorithms, computational biology, and many other topics. This collection provides valuable insights into current developments and trends in algorithm research and is a significant resource for professionals and students in computer science and related disciplines.