This Festschrift is dedicated to Sjouke Mauw on the occasion of his 65th birthday. Its title reflecting not only two of his central academic pursuits but his playful nature. Sjouke s academic career started with a focus on formal specifications which expanded to encompass the field of digital security. His oeuvre is marked by a rigorously formal approach to problems, which has enabled him not only to reveal technical subtleties but also to anticipate important conceptual shifts.
In addition to his successes in research, Sjouke has always been deeply committed to teaching, taking care in nurturing and challenging students to develop. Many of them have contributed to this volume, and collectively this reflects his excellent impact as a researcher, educator, and mentor.