Afifth edition of Essentials of Anatomy and Physiology is very gratifying, and again we have the pleasure of thanking all of our colleagues in teaching who continue to use our textbook and of welcoming new teachers and students. Changes have been made in the content of the book to keep the material as current as possible. Most of these are small changes, such as a new cell organelle, the proteasome, in Chapter 3; a clarification of blood cell production in Chapters 4, 11, and 14; and the addition of concentric and eccentric contractions to Chapter 7. The most extensive revision is in Chapter 14: The section on immunity has been rewritten using the categories of innate and adaptive immunity