This special issue contains selected papers of the 20th International Conference on Rewriting Techniques and Applications (RTA 2009), which was held from June 29 to July 1, 2009, in Brasília, Brazil as part of 5th International Conference on Rewriting, Deduction, and Programming (RDP 2009).

RTA is the major forum for the presentation of research on all aspects of rewriting. Previous RTA conferences were held in Dijon (1985), Bordeaux (1987), Chapel Hill (1989), Como (1991), Montreal (1993), Kaiserslautern (1995), Rutgers (1996), Sitges (1997), Tsukuba (1998), Trento (1999), Norwich (2000), Utrecht (2001), Copenhagen (2002), Valencia (2003), Aachen (2004), Nara (2005), Seattle (2006), Paris (2007), and Hagenberg (2008).

Ten out of the the 26 papers presented at the conference were invited to submit an extended version to this special issue, including the two papers that had received the Best Paper Award at the conference: An Explicit Framework for Interaction Nets by Marc de Falco, and From Outermost to Context-Sensitive Rewriting by J&oumj;rg Endrullis and Dimitri Hendriks. Each submitted paper was refereed by at least two expert referees, at least one of them not having been a referee of the conference submission. Seven papers were finally accepted for inclusion in this special issue.

Our thanks go to the members of the Program Committee of RTA 2009 and their subreferees, as well as to all those who served in addition as reviewers for this special issue. We would also like to thank Benjamin Pierce, Managing Editor of LMCS, for having invited us to edit this special issue, and for his assistance and guidance.