The full content of the 21^st edition of Harrison’s Principles of Internal Medicine is now online at AccessMedicine and AccessPharmacy. The 21^st edition represents the full spectrum of internal medicine, and has been updated to reflect practice-changing developments and guidelines emerging over the past three years. Of particular note is our coverage of COVID-19, in the trusted and balanced Harrison’s style that students and clinicians worldwide have come to rely upon.

There are many brand new chapters in the 21^st edition, including those on cutting edge topics like AI and Precision Medicine, as well as a new chapter on Diagnostic Errors and how to reduce them. Also of note is a brand new chapter on the regulation and dysregulation of the immune system: a tour de force from world-leading authorities on one of the hottest topics in biomedicine. Our section on approach to signs and symptoms and associated clinical reasoning is comprehensive and useful for building diagnostic schema. Disease-oriented chapters reflect the current state of the science, from cardiology to infectious diseases to endocrinology to neurology and more.

Chapter 1 of Harrison’s is a terrific overview of what it means to practice and learn medicine and care for patients, and we are providing it here for all to read.