This proceedings volume contains the invited papers and the contributed papers th accepted for presentation at the 9 International Symposium on Experimental Algorithms (SEA 2010), that was held at the Continental Terme Hotel, Ischia (Naples), Italy, during May 20-22, 2010. PrevioussymposiaoftheserieswereheldinRiga(2001),MonteVerita(2003), RiodeJaneiro(2004),Santorini(2005),Menorca(2006),Rome(2007),CapeCod (2008), and Dortmund (2009). Seventy-three papers were submitted by researchers from 19 countries. Each paper was reviewed by three experts among the Program Committee members and some trusted external referees. At least two reviewers were from the same or closely related discipline as the authors. The reviewers generally provided a high-quality assessment of the papers and often gave extensive comments to the authors for the possible improvement of the presentation. The submission and review process was supported by the ConfTool conference management software and we are thankful to Harald Weinreich for letting us use it. The Program Committee selected 40 regular papers for presentation at the conference. In addition to the 40 contributed papers, this volume includes two invitedpapersrelatedtocorrespondingkeynotetalks:GiuseppeF.Italiano(U- versity of Rome "Tor Vergata,"Italy)spoke on "ExperimentalStudy of Resilient Algorithms and Data Structures" and PanosM. Pardalos(University of Florida, USA) spoke on "Computational Challenges with Cliques, Quasi-Cliques and Clique Partitions in Graphs." Many people and organizations contributed to SEA 2010. We are parti- larlygrateful for the patronageand ?nancial support of the University of Naples "FedericoII"andtheDepartmentofMathematicsandApplications"R.Cacci- poli,"and for the ?nancial support of GNCS (Gruppo Nazionale per il Calcolo Scienti?co) - INdAM (Istituto Nazionale di Alta Matematica).