The disease that bears William J. Baumol’s name is not what led to his death on May 4 2017 at age 95, but it is what cemented his legacy as one of the pre-eminent economists of the 20th century. Professor Baumol, who taught for decades at both Princeton University and New York University, identified what has come to be known as Baumol’s cost disease. This so-called affliction is actually a critically important economic insight that explains why the cost of services, like haircuts and college educations, rises faster than the cost of goods, like T-shirts. Professor Baumol was “one of the great economists of his generation,” Joseph Stiglitz, a Nobel Prize-winning economist at Columbia University, said in an interview. William Jack Baumol was born in the South Bronx on Feb. 22, 1922, to Solomon and Lillian Baumol, immigrants from Eastern Europe. He served in the Army in World War II and got a job at the Agriculture Department, where he worked on allocating grain supplies to starving countries. He graduated from City College, where he met his future wife, Hilda Missel, and he enrolled in the London School of Economics in 1947 after initially being rejected. Less than six weeks after school started, he was hired to become a member of the faculty. He landed at Princeton in 1949 and remained there for the rest of his life as a researcher and eventually as a professor emeritus. He joined the faculty of N.Y.U. in 1971 and retired from there only in 2014. His death, at his home in New York City, was confirmed by his son, Daniel. Other survivors include his wife, a former management consultant; a daughter, Jasmine Wolf; and two grandchildren.