The 2020 S. Barry Cooper Prize is awarded to Bruno Courcelle for his work on the definability of graph properties in Monadic Second Order Logic, through a sequence of seminal papers and a book (joint with Joost Engelfriet). This forms an outstanding example of theory building, bringing together logic, computability, graph grammars, and various notions of graph width (tree-width, clique-width and rank-width) and opening new avenues in our understanding of graph structure theory and the computability and complexity of graph algorithms. Besides its foundational character, the work has had great impact on a number of areas of computer science, including in parameterized algorithmics, verification and other areas, and has influenced a generation of researchers in this field. It has straddled the divide between the logical and algorithmic aspects of theoretical computer science. The award ceremony will take place on July 2nd during the Computability in Europe 2020 conference. Registration at the conference (https://www.acie.eu/cie-conference-series/cie2020) is mandatory to attend, but it is without fees. The Barry Cooper Prize Committee 2019-2023 Anuj Dawar Peter Van Emde Boas Yuri Gurevitch Mariya Soskova Paola Bonizzoni