This book constitutes the refereed proceedings of the 11th Scandinavian Workshop on Algorithm Theory, SWAT 2008, held in Gothenborg, Sweden, in July 2008. The 36 revised full papers presented together with 2 invited lectures were carefully reviewed and selected from 111 submissions. Papers were solicited for original research on algorithms and data structures in all areas, including but not limited to: approximation algorithms, computational biology, computational geometry, distributed algorithms, external-memory algorithms, graph algorithms, online algorithms, optimization algorithms, parallel algorithms, randomized algorithms, string algorithms and algorithmic game theory.