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, one of the discoverers 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.
The Priestley Medal is among the most distinguished awards in the chemical sciences, behind the Wolf Prize in Chemistry and the Nobel Prize in Chemistry. Consequently, it is commonly awarded to scientists who are advanced in their fields, as it is intended to commemorate lifetime achievement. When the ACS started presenting the Priestley Medal in 1923, they intended to award it every three years. This continued until 1944, when it became an annual award.