Penicillins (P, PCN or PEN) are a group of antibiotics originally obtained from Penicillium moulds, principally P. chrysogenum and P. rubens. Most penicillins in clinical use are chemically synthesised from naturally-produced penicillins. A number of natural penicillins have been discovered, but only two purified compounds are in clinical use: penicillin G (intramuscular or intravenous use) and penicillin V (given by mouth). Penicillins were among the first medications to be effective against many bacterial infections caused by staphylococci and streptococci. They are members of the -lactam antibiotics. They are still widely used today for different bacterial infections, though many types of bacteria have developed resistance following extensive use.

About 10% of people report that they are allergic to penicillin; however, up to 90% of this group may not actually be allergic. Serious allergies only occur in about 0.03%. Those who are allergic to penicillin are most often given cephalosporin C (another -lactam antibiotic) because there is only 10% crossover in allergy between the penicillins and cephalosporins.Penicillin was discovered in 1928 by Scottish scientist Alexander Fleming as a crude extract of P. rubens. Fleming's student Cecil George Paine was the first to successfully use penicillin to treat eye infection (ophthalmia neonatorum) in 1930. The purified compound (penicillin F) was isolated in 1940 by a research team led by Howard Florey and Ernst Boris Chain at the University of Oxford. Fleming first used the purified penicillin to treat streptococcal meningitis in 1942. For the discovery, Fleming shared the 1945 Nobel Prize in Physiology or Medicine with Florey and Chain.

Several semisynthetic penicillins are effective against a broader spectrum of bacteria: these include the antistaphylococcal penicillins, aminopenicillins and the antipseudomonal penicillins.

Orvan Walter Hess (June 18, 1906 – September 6, 2002) was an American physician noted for his early use of penicillin and the development of the fetal heart monitor.

Hess was born in Baoba, Pennsylvania. At the age of two, after his mother's death, the family moved to Margaretville, New York where he grew up. Hess was inspired by Doctor Gordon Bostwick Maurer—who started Margaretville's first hospital in 1925— to study medicine. He married Dr. Maurer's sister, Carol Maurer, in 1928.

Hess went to Lafayette College and was graduated in 1927, and received his MD from the University at Buffalo. He completed an internship at Children's Hospital in Buffalo, New York and became an obstetrician and gynecologist.

For most of his career, Hess practiced at Yale-New Haven Hospital, interrupted by World War II service as a surgeon in the 48th Armored Medical Battalion attached to the 2nd Armored Division in the invasions of North Africa, Sicily and Normandy.

He was clinical professor of obstetrics and gynecology at the Yale School of Medicine. He also served as president of the Connecticut State Medical Society, and director of health services for the Connecticut Welfare Department. Hess died in New Haven at the age of 96.

Hess was predeceased by his wife Carol in 1998. He is survived by two daughters, Dr. Katherine Halloran of Lexington, and Carolyn Westerfield of Hamden; five grandchildren; and five great-granddaughters.