This text attempts to introduce the molecular biology of cell membranes to students and professionals of diverse backgrounds. Although several membrane biology books are available, they do not integrate recent knowledge gained using modern molecular tools with more traditional membrane topics. Molecular techniques, such as cDNA cloning and x-ray diffraction, have provided fresh insights into cell membrane structure and function. The great excitement today, which I attempt to convey in this book, is that molecular details are beginning to merge with physiological responses. In other words, we are beginning to understand precisely how membranes work. This textbook is appropriate for upper-level undergraduate or beginning graduate students. Readers should have previous or concurrent coursework in biochemistry; prior studies in elementary physiology would be helpful. I have found that the presentation of topics in this book is appropriate for students of biology, biochemistry, biophysics and physiology, chemistry, and medicine. This book will be useful in courses focusing on membranes and as a supplementary text in biochemistry courses. Professionals will also find this to be a useful resource book for their personal libraries.