Barry James Marshall, AC, FRS, FAA (born 30 September 1951) is an Australian physician, Nobel Prize laureate in Physiology or Medicine, and Professor of Clinical Microbiology at the University of Western Australia. Marshall and Robin Warren showed that the bacterium Helicobacter pylori (H. pylori) is the cause of most peptic ulcers, reversing decades of medical doctrine holding that ulcers were caused by stress, spicy foods, and too much acid.

Marshall was born in Kalgoorlie, Western Australia and lived in Kalgoorlie and Carnarvon until moving to Perth at the age of eight. His father held various jobs, and his mother was a nurse. He is the eldest of four siblings. He attended Newman College and the University of Western Australia, where he received a Bachelor of Medicine and Surgery in 1975. He married his wife Adrienne in 1972 and has four children.

John Robin Warren AC (born 11 June 1937 in Adelaide) is an Australian pathologist, Nobel Laureate and researcher who is credited with the 1979 re-discovery of the bacterium Helicobacter pylori, together with Barry Marshall.

He received his MBBS degree from the University of Adelaide, having completed his high school education at St Peter's College, Adelaide. In 1967 he was admitted into the Royal College of Pathologists of Australasia and became a senior pathologist at the Royal Perth Hospital where he spent the majority of his career. At the University of Western Australia with his colleague Barry J. Marshall, Warren proved that the bacterium is the cause of stomach ulcers. Warren helped develop a convenient diagnostic test (¹⁴C-urea breath-test) for detecting H. pylori in ulcer patients. In 2005, Warren and Marshall were awarded the Nobel Prize in Medicine.

He married Winifred Theresa Warren (née Williams) and together they had six children. Winifred Warren went on to become an accomplished psychiatrist.