Sir David W. C. MacMillan is a Scottish-American organic chemist and the James S. McDonnell Distinguished University Professor of Chemistry at Princeton University, where he chaired the Department of Chemistry from 2010 to 2015 and founded the Princeton Catalysis Initiative. He shared the 2021 Nobel Prize in Chemistry with Benjamin List for the development of asymmetric organocatalysis. MacMillan introduced the concept of using small chiral secondary amines to form reactive iminium ions, and he coined the term 'organocatalysis' itself; his imidazolidinone catalysts — widely known as MacMillan catalysts — enabled enantioselective Diels–Alder, Friedel–Crafts, and cascade reactions that are now textbook methods. He subsequently pioneered visible-light photoredox catalysis, merging photochemistry with organocatalysis and metal catalysis to forge carbon–carbon bonds under mild conditions, a strategy now heavily used by the pharmaceutical industry for late-stage functionalization and library synthesis. Born in Bellshill to a steelworker father, MacMillan earned his undergraduate degree at the University of Glasgow and his PhD at the University of California, Irvine, held faculty positions at Berkeley and Caltech before Princeton, and used his share of the Nobel Prize to establish the May and Billy MacMillan Foundation supporting education in his native Scotland.