The third edition of this text has been completely rewritten and revised. It is intended for first- and second-year undergraduates in chemistry taking physical chemistry courses, and for undergraduates in other science and engineering subjects that require an understanding of chemistry. The author gives more attention to the solid and liquid states than is found in other texts on this subject, and introduces topics such as computer simulation and quasicrystals. Each chapter concludes with a set of problems, to which there are solution notes, designed to lead the reader to familiarity with the subject and its application in new situations. Computer programs designed to assist the reader are downloadable from the World Wide Web, from the time of publication. Detailed solutions to the problems will also be available via the World Wide Web. See http://www.cup.cam.ac.uk/stm/laddsolutions.htm. This modern text on physical chemistry will be of interest to undergraduate students in chemistry and also students in other areas of science and engineering requiring a familiarity with the subject.