Thisvolumecontainsthe66contributedpapersandabstractsofthethreeinvited lecturespresentedatthe11thAnnualEuropeanSymposiumonAlgorithms(ESA 2003), held in Budapest, September 16-19, 2003. The papers in each section of the proceedings are arranged alphabetically. The three distinguished invited ´ speakers were Bernard Chazelle, Roberto Tamassia, and Eva Tardos. Forthesecondtime, ESAhadtwotracks, withseparateprogramcommittees, which dealt respectively with: The design and mathematical analysis of algorithms (the "Design and Analysis" track); Real-world applications, engineering, and experimental analysis of al- rithms (the "Engineering and Applications" track). Previous ESAs were held at Bad Honnef, Germany (1993); Utrecht, The Neth- lands (1994); Corfu, Greece (1995); Barcelona, Spain (1996); Graz, Austria (1997); Venice, Italy (1998); Prague, Czech Republic (1999); Saarbruc ] ken, Ger- ? many (2000); Arhus, Denmark (2001), and Rome, Italy (2002). The predecessor to the Engineering and Applications track of ESA was the annual Workshop on Algorithm Engineering (WAE). Previous WAEs were held in Venice, Italy (1997), Saarbruc ] ken, Germany (1998), London, UK (1999), Saarbruc ] ken, Ger- ? many (2000), Arhus, Denmark (2001), and Rome, Italy (2002) . The proceedings of the previous ESAs were published as Springer-Verlag's LNCS volumes 726, 855, 979, 1284, 1461, 1643, 1879, 2161, and 2461. The p- ceedings of the WAEs from 1999 onwards were published as Springer-Verlag's LNCS volumes 1668, 1982, and 2141.