A well known name among mathematicians dealing with probability theory. More than 150 mathematical notions bear his name, and here are just a few of them: Feller's process, Feller's transition function, Feller's semigroup, Feller's property.
He is best known for his monograph "An Introduction to Probability Theory and its Applications", Volumes I and II, on 1153 pp., translated into Russian, Chinese and Polish. They are considered among the best mathematical textbooks written in the 20th century. He was president of the Institute of Mathematical Statistics and he was a member of the Royal Statistical Society in the UK. He was elected to the National Academy of Sciences (United States) and was also a member of the American Academy of Arts and Sciences. He was awarded the 1969 National Medal for Science but died shortly before the presentation was to be made. His wife received the medal on his behalf.