There has been a convergence in recent years of people from the physical and biological sciences and from various engineering disciplines who are interested in analyzing the electrical activity of nerve and muscle quantita- tively. Various courses have been established at the graduate level or final-year undergraduate level in many universities to teach this subject matter, yet no satisfactory short text has existed. The present book is an attempt to fill this gap, and arises from my experience in teaching this material over the past fifteen years to students on both sides of the Atlantic. Although covering a wide range of biophysi- cal topics from the level of single molecules to that of complex systems, I have attempted to keep the text relatively short by considering only examples of the most general interest. Problems are included whenever possible at the end of each chapter so the reader may test his understand- ing of the material presented and consider other examples which have not been included in the text.