Thisvolumecontainsthepapersofthe20thInternationalConferenceonRewr- ing Techniques and Applications (RTA 2009), which was held from June 29 to July 1, 2009, in Bras' ?lia, Brazil as part of the 5th International Conference on Rewriting, Deduction, and Programming (RDP 2009) together with the Int- national Conference on Typed Lambda Calculi and Applications (TLCA 2009), the International School on Rewriting (ISR 2009), the 4th Workshop on Logical and Semantic Frameworks with Applications (LSFA 2009), the 10th Inter- tional Workshop on Rule-Based Programming (RULE 2009), the 8th Inter- tional Workshop on Functional and (Constraint) Logic Programming (WFLP 2009), the 9th International Workshop on Reduction Strategies in Rewriting and Programming (WRS 2009), and the annual meeting of the IFIP Working Group 1.6 on term rewriting. RTA is the major forum for the presentation of research on all aspects of rewriting.PreviousRTAconferenceswereheldinDijon(1985),Bordeaux(1987), Chapel Hill (1989), Como (1991), Montreal (1993), Kaiserslautern (1995), R- gers (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).

Constraints provide a declarative way of representing infinite sets of data. They are well suited for combining different logical or programming paradigms as has been known for constraint logic programming since the 1980s and more recently for functional programming. The use of constraints in automated deduction is more recent and has proved to be very successful, moving the control from the meta-level to the constraints, which are now first-class objects.
This monograph-like book presents six thoroughly reviewed and revised lectures given by leading researchers at the summer school organized by the ESPRIT CCL Working Group in Gif-sur-Yvette, France, in September 1999. The book offers coherently written chapters on constraints and constraint solving, constraint solving on terms, combining constraint solving, constraints and theorem proving, functional and constraint logic programming, and building industrial applications.