This book constitutes the refereed proceedings of the 21st International Colloquium on Trees in Algebra and Programming, CAAP '96, held in Linköping, Sweden, in April 1996.The 21 full revised papers presented were selected from 44 submissions; also included are invited papers by Samson Abramsky and Frank Pfenning. In keeping with the tradition of the CAAP conferences, this volume covers algebraic, logical, and combinatorial properties of discrete structures and their application to computer science, in particular to the theory of computation and to programming theory.