This book provides a non-mathematical and highly visual account of the concepts of quantum mechanics widely encountered in chemistry and related disciplines. Entries - ordered alphabetically - range in length from one paragraph to several pages, and describe the physical significance of each topic with simplicity and precision. Each entry is extensively cross-referenced and ends with a guide to sources of further information. This new work continues the tradition of the first edition, which quickly established itself among students and teachers as a rich source of clear, concise explanations of quantum concepts. The text has been completely rewritten - all topics have been brought up to date, and there are many more illustrations and numerous new entries. The book should be helpful to students for its brief non-mathematical summaries of key concepts, to instructors as a source of explanations, illustrations, and references, and to all practicing scientists who need a succinct reminder of a topic. The work is aimed principally at students and teachers of chemistry, and practising chemists who are not quantum specialists; it should also prove useful to physicists and biologists, and an accessible account of quantum ideas for the interested general reader.