Knowles is best known for his history of early English monasticism, "The Monastic Order in England: A History of Its Development from the Times of St. Dunstan to the Fourth Lateran Council, 940–1216" (1940). His three-volume work, "The Religious Orders in England" (1948–1959), is also highly regarded by scholars in English medieval ecclesiastical history. In 1962 he published a textbook, "The Evolution of Medieval Thought" (2nd ed. 1988), that "dominated medieval history courses in U.S. colleges for a quarter of a century". He has been criticised for excluding nunneries from consideration in "Medieval Religious Houses" on the grounds that there was insufficient evidence to draw on (a lack remedied in more recent scholarship).