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...