The Mustard Operation is a surgical technique to correct a heart condition called the transposition of the great arteries (TGA). TGA is a birth defect in which the placement of the two arteries, the pulmonary artery, which supplies deoxygenated blood to the lungs, and the aorta, which takes oxygenated blood to the body are switched. William Thornton Mustard developed the operation later named for him and in 1963 operated on an infant with TGA, and ameliorated the condition, at the Hospital for Sick Children in Toronto, Canada. Afterwards, the Mustard Operation became the primary form of corrective surgery for TGA, until the arterial switch operation largely replaced the Mustard Operation by the late 1990s. The Mustard Operation enabled surgeons to correct TGA in infants born with the life-threatening anomaly, increasing their life spans and quality of life.

Étienne-Louis Arthur Fallot was a physician working in France during the late nineteenth century who studied and described the four cardiac anatomical defects that cause the congenital anomaly known as the Tetralogy of Fallot. Those four heart defects result in deoxygenated blood recirculating through the body, giving the skin a blue-like color, a process called cyanosis. Numerous physicians and researchers before Fallot had identified and described the anatomical cardiac defects that would eventually be included in the Tetralogy. However, Fallot was among the first to note that the four anatomical heart defects tended to occur together. Through autopsy investigations, Fallot established the Tetralogy as one unified pathology rather than four unrelated anatomical abnormalities, providing a basis for the eventual surgical treatment of the condition, which affects approximately four out of every one thousand births worldwide.