Computational fluid dynamics (CFD) has become a popular tool to develop and design membrane systems and processes. CFD enables the simulation of flowing or stationary fluids and multiphase systems, but also to the description of mass conversions and reaction processes and their coupling among each other. The following chapter provides an overview of CFD of microfiltration (MF) and ultrafiltration (UF) processes including theory and modeling techniques for both single- and multiphase flow. The chapter includes general information about MF/UF membranes, the state of the art of CFD in MF/UF membranes, modeling details, and comments for the future.