The Priestley Medal is the highest honor conferred by the American Chemical Society (ACS) and is awarded for distinguished service in the field of chemistry. Established in 1922, the award is named after Joseph Priestley, the discoverer of oxygen who immigrated to the United States of America in 1794. The ACS formed in 1876, spearheaded by a group of chemists who had met two years previously in Priestley's home.