This Festschrift, dedicated to Frits W. Vaandrager on the occasion of his 60th birthday, contains papers written by many of his closest collaborators. Frits has been a Professor of Informatics for Technical Applications at Radboud University Nijmegen since 1995, where his research focuses on formal methods, concurrency theory, verification, model checking, and automata learning.   The volume contains contributions of colleagues, Ph.D. students, and researchers with whom Frits has collaborated and inspired, reflecting a wide spectrum of scientific interests, and demonstrating successful work at the highest levels of both theory and practice.