This volume is dedicated to Professor Arto Salomaa on the occasion of his 60th birthday. The 32 invited papers contained in the volume were presented at the festive colloquium, organized by Hermann Maurer at Graz, Austria, in June 1994; the contributing authors are well-known scientists with special relations to Professor Salomaa as friends, Ph.D. students, or co-authors. The volume reflects the broad spectrum of Professor Salomaa's research interests in theoretical computer science and mathematics with contributions particularly to automata theory, formal language theory, mathematical logic,...