William Feller (1906 - 1970) was one of the most distinguished figures in probability theory of the 20th century. His work contains seminal contributions to the areas of Stochastic Processes (in particular: “Feller” processes, boundary theory, analytic treatment of stochastic processes), Limit theorems for random variables (Central Limit Theorem, Fluctuation Theory) and Applications of Stochastic Processes to Mathematical Biology. Feller was a prolific writer and even his “minor” contributions are still quoted nowadays, e.g. Contributions to measure theory (differentiation of measures), Contributions to geometry (Feller-Alexandrov-Busemann curvature). Among his almost twenty doctoral students one finds some of the most distinguished mathematicians, including Patrick Billingsley, Henry McKean, Hale Trotter, Frank Knight, David Freedman, Martin Silverstein, Lawrence Shepp.