This book constitutes the refereed proceedings of the 20th International Conference on Rewriting Techniques and Applications, RTA 2009, held in Bras?lia, Brazil, during June 29 - July 1, 2009.

The 22 revised full papers and four system descriptions presented were carefully reviewed and selected from 59 initial submissions. The papers cover current research on all aspects of rewriting including typical areas of interest such as applications, foundational issues, frameworks, implementations, and semantics.