William Gropp’s interest is in the use of high performance computing to solve problems that are too hard for other techniques. He concentrates in two areas: the development of scalable numerical algorithms for partial differential equations (PDEs), especially for the linear systems that arise from approximations to PDEs, and the development of programming models and systems for expressing and implementing highly scalable applications. In each of these areas, he leads the development of software that has been widely adopted. PETSc is a powerful numerical library for the solution of linear and nonlinear systems of equations. MPI is the mostly widely used parallel programming system for large scale scientific computing. The MPICH implementation of MPI is one of the most widely used and is the implementation of choice for the world’s fastest machines.