Marian Breland Bailey, born Marian Ruth Kruse (December 2, 1920 – September 25, 2001) and nicknamed "Mouse", was an American psychologist, an applied behavior analyst who played a major role in developing empirically validated and humane animal training methods and in promoting their widespread implementation. She and her first husband, Keller Breland (1915–1965), studied at the University of Minnesota under behaviorist B. F. Skinner and became "the first applied animal psychologists." Together they wrote the book Animal Behavior which was first published in 1966, after Keller's death.