Horng-Tzer Yau is a mathematician who applies mathematical insights and analysis to the explanation of important physical processes. His work focuses on reinterpreting descriptive models of large-scale physical behavior within the context of statistical mechanics. Early in his career, Yau focused on describing in quantum mechanical terms the stability of matter in complex (many-body) systems. This work produced sound mathematical support for earlier astrophysical theories on the limits of stellar stability. He then shifted the direction of his research toward explaining the macroscopic properties of fluids based on the microscopic behavior of their constituent particles—the hydrodynamic limit. He developed the novel concept of “relative entropy” to derive descriptive formulae of fluid behavior (for example, Navier-Stokes) from basic principles of statistical mechanics. Although the problems that Yau works on are rooted in physical phenomena, he has made important contributions to fundamental mathematics in several areas: probability theory, nonlinear partial differential equations, spectral theory, and dynamical systems theory. Yau is a professor of mathematics at Harvard University. He served previously as a professor of mathematics at Stanford University (2003-2005) and as a professor at the Courant Institute of Mathematical Sciences at New York University (1988-2003). Yau received a B.S. (1981) from the National Taiwan University and a Ph.D. (1987) from Princeton University.