Since the first edition of this series, we have witnessed the birth of “molecular medicine,” using biochemistry, cell biology, and genetics to diagnose and treat disease. Consequently, the basic sciences are becoming more important to the practice of medicine. This puts a new pressure on the student—to understand the basis of molecular medicine and the molecular sciences. I still think that it’s easier to remember things that you understand, things that make sense. That’s the idea behind the Basic Concepts series and that’s why I have been so pleased with the expansion of the Basic Concepts series beyond Biochemistry. The revisions in the second edition include two new chapters, “Membranes and Membrane Proteins” and “Signal Transduction Pathways.” These topics are related to the explosion of new information about cell signaling and signal transduction pathways. In addition, I’ve added some tables of information that I think will be helpful in seeing the big picture (and remembering some of the more important details). As before, the major topics and things to remember are set off in boxes so that if you already know everything in the box, you can skip the rest of the section. The first page of each chapter presents an index. A title-summary box for each section presents a short summary and memory jogger intended to be helpful for review.