In August 1928, Alexander Fleming returned from a vacation to his usually messy, disordered laboratory. In one of the Petri dishes that had not been touched by the Lysol, he noticed an unusual phenomenon: separate colonies of staphylococci and, near the dish's edge, a colony of mold approximately 20 mm in diameter. The finding proved to be a watershed in the history of medicine. This discovery lay dormant for some time before other researchers took up the challenge to investigate its clinical possibilities. Two investigators at Oxford, Sir Howard Walter Florey and Ernst Boris Chain, brought penicillin's potential for medical use to fruition and, along with Fleming, shared the 1945 Nobel Prize for Medicine. The discovery and development of penicillin represent one of the most important developments in the annals of medical history. This article presents a brief overview of the events that occurred in the progress from discovery to implementation as a therapeutic agent.