The book "Categorical Structure of Closure Operators" by D. Dikranjan and W. Tholen has appeared in the "Mathematics and Its Applications" series of Kluwer Academic Publishers (Dordrecht, Boston, London 1995; ISBN 0-7923-3772-7). Abstract: The book provides a comprehensive categorical theory of closure operators, with applications to topological and uniform spaces, groups, R-modules, fields and topological groups, as well as to partially ordered sets and graphs. In particular, closure operators are used to give solutions to the epimorphism and cowellpoweredness problem in many concrete categories. The material is illustrated with many examples and exercises, and open problems are formulated in order to stimulate further research. -- Walter Tholen Department of Mathematics and Statistics York University, North York, Ont. Canada M3J 1P3 tel. (416) 736 5250 fax. (416) 736 5757