This book containing conjectures is meant to occupy my husband, Guido Mislin, during the long years of his retirement. I view this project with appreciation, since I was wondering how that mission was to be accomplished. In the thirty-five years of our acquaintance, Guido has usually kept busy with his several jobs, our children and occasional, but highly successful projects that he has undertaken around the house. The prospect of a Guido unleashed from the ETH; unfettered by professional duties of any sort, wandering around the world as a free agent with, in fact, nothing to do, is a prospect that would frighten nations if they knew it was imminent. I find it a little scary myself, so I am in a position to appreciate the existence of this project from the bottom of my heart.