: 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.