Why would you read this preface? As we start thinking what to write here, we wonder who is going to read these words. Fromourperspective–thatofwritersaddressinganaudienceofreaders–you are most likely Willem-Paul de Roever. Willem: our main motivation in putting together this Festschrift is to honor you on the occasion of your retirement. In terms of scienti?c ancestry, you are a father to two of us, and a grandfather to 1 the third , and you have had a profound impact on our formation as computer scientists.Atthepersonallevel,weknowyouasakind-hearted,generousperson. We are grateful to know you in these ways, and hope to have encounters with you in many years to come. AnotherlikelypossibilityisthatyouareCorinneorJojanneke,wifeordau- ter of Willem; the two strong pillars on which so much in his life is founded. You share the honor,respect, and love that went into the writing, as will be ackno- edged by those contributing authors that know you – which are almost all. Also, we would like to thank you for your help in sending us photographs for inclusion in this book, and for your encouragement. The next option is that you are one of the contributing authors. In this case you may wonder why it took us so long to get this work published. After all, wasn’tit“almostdone”alreadyattheretirementeventinJuly2008?Theanswer is twofold: we gave everyone ample time to revise their submissions in line with the recommendations by the referees; and we ourselves took ample time to put everything together. Our hope is that this will be visible in the quality of the ?nal result.