Sir William Osler (1849-1919) was a Canadian physician. He has been called one of the greatest icons of modern medicine. Osler was a pathologist, educator, bibliophile, historian, author, and renowned practical joker. Perhaps Osler's greatest contribution to medicine was to insist that students learned from seeing and talking to patients and the establishment of the medical residency. This latter idea spread across the English-speaking world and remains in place today in most teaching hospitals. Through this system, doctors in training make up much of a hospital's medical staff. Osler was a prolific author and public speaker and his public speaking and writing were both done in a clear, lucid style. His most famous work, 'The Principles and Practice of Medicine' quickly became a key text to students and clinicians alike. It continued to be published in many editions until 2001 and was translated into many languages. Osler's essays were important guides to physicians. The title of his most famous essay, Aequanimitas, espousing the importance of imperturbability, is the motto on the Osler family crest and is used on the Osler housestaff tie and scarf at Hopkins.