For the first of our series Conversations with Giants in Medicine, we spoke with Harold E. Varmus, MD, who has been the director of the National Cancer Institute since July 2010. He has previously served as director of the National Institutes of Health (1993–1999) and as president of the Memorial Sloan-Kettering Cancer Center (2000–2010). Among his many awards, Dr. Varmus was awarded the 1989 Nobel Prize in Physiology or Medicine together with J. Michael Bishop in recognition of their discovery of the cellular origin of retroviral oncogenes.