This volume contains the proceedings of the Fifteenth Colloquium on Trees in Algebra and Programming. The papers selected present new research results and cover the following topics: - Logical, algebraic and combinatorial properties of discrete structures (strings, trees, graphs, etc.), including the theory of formal languages considered as that of sets of discrete structures and the theory of rewriting systems over these objects. - Application of discrete structures in computer science, including syntax and semantics of programming languages, operational semantics, logic programming,...