This Festschrift is dedicated to Jan Willem Klop on the occasion of his 60th birthday. The volume comprises a total of 23 scientific papers by close friends and colleagues, written specifically for this book. The papers are different in nature: some report on new research, others have the character of a survey, and again others are mainly expository. Every contribution has been thoroughly refereed at least twice. In many cases the first round of referee reports led to significant revision of the original paper, which was again reviewed. The articles especially focus upon the lambda calculus, term rewriting and process algebra, the fields to which Jan Willem Klop has made fundamental contributions.