In 1900 William Osler established a friendship with Harvey Cushing that encompassed the personal and professional aspects of their lives for over two decades. Their shared participation in the covenant of medicine shaped an intense friendship and mentoring relationship that profited both individuals immeasurably. The choice of Cushing as the recipient of Osler's mentoring had its origins in their rearing, avocations, and in the way of life that they shared. In Cushing, Osler identified a surrogate son who joined with him in defining the course of medicine and surgery over the next century.