The fourth edition contains again various revisions and updates throughout the whole book. There are many new comments, formulations and arguments, several new references, and three minor error corrections (regarding page 22, 112 and 146 of the third edition). This time I am grateful to David Atkinson (for a very useful discussion of radiation damping – Sect. 2. 3), to Larry Schulman (for comments on the problem of simultaneous arrows of time – Sect. 3. 1. 2), and to Paul Sheldon (for a discussion of the compatibility of closed time-like curves with quantum theory – Chap. 1). The most efficient help came from John Free, who carefully edited the whole fourth edition (not only for matters of English language). Heidelberg, April 2001 H. D. Zeh Preface to the Third Edition The third (1999) edition of the Direction of Time offered far more revisions and additions than the second one in 1992. During the seven years in between, several fields of research related to the arrow of time had shown remarkable progress. For example, decoherence proved to be the most ubiquitous man- ifestation of the quantum arrow, while articles on various interpretations of quantum theory (many of them with inbuilt time-asymmetric dynamical aspects) can and do now regularly appear in reputed physics journals. There- fore, most parts of Chap. 4 were completely rewritten and some new sections added, while the second part of Chap.