This book constitutes the refereed proceedings of the 13th Annual International Conference on Research in Computational Molecular Biology, RECOMB 2009, held in Tucson, Arisona, USA in May 2009. The 37 revised full papers presented were carefully reviewed and selected from 166 submissions. As the top conference in computational molecular biology, RECOMB addresses all current issues in algorithmic, theoretical, and experimental bioinformatics such as molecular sequence analysis, recognition of genes and regulatory elements, molecular evolution, protein structure, structural genomics, gene expression, gene networks, drug design, combinatorial libraries, computational proteomics, as well as structural and functional genomics.