This book constitutes the refereed proceedings of the 20th Annual Symposium on Combinatorial Pattern Matching, CPM 2009, held in Lille, France in June 2009. The 27 revised full papers presented together with 3 invited talks were carefully reviewed and selected from 63 submissions. The papers address all areas related to combinatorial pattern matching and its applications, such as coding and data compression, computational biology, data mining, information retrieval, natural language processing, pattern recognition, string algorithms, string processing in databases, symbolic computing and text searching.