The third edition of this highly successful book has been extensively revised and restructured to incorporate the many recent advances in the subject, including new information on the properties of single ionic channels and the molecular biology of membrane proteins. There are many new illustrations and numerous references to recent work. The essential philosophy of the book remains the same: fundamental concepts are clearly explained and key experiments are examined in some detail. The contents of the book that was so successfully launched in 1971 are now appropriate to the challenges of the 1990s. The book is primarily intended for use by students of physiology, biophysics, neuroscience or zoology, and will be useful to those beginning research, and to scientists of related disciplines.