Membrane filtration is frequently used in animal cell culture for bioreactor harvesting, protein concentration, buffer exchange, virus filtration, and sterile filtration. A variety of membrane materials and pore sizes ranging from loose microfiltration membranes to tight ultrafiltration membranes, which reject small proteins, are frequently found in a purification train. While all of these operations make use of the same size-based separation principle, the actual methods of operation vary significantly. Microfiltration is often the first of the unit operations in the purification train. Microfiltration membranes have pores in the micrometer size range. Microfiltration is used to remove cells and cell debris. This chapter begins by describing tangential flow microfiltration. A typical method of operation is included. Concentration of the product and buffer exchange is often required toward the end of the purification train. Ultrafiltration membranes are used for both operations. The theory of tangential flow ultrafiltration is briefly described followed by a typical method of operation. Today, large-pore ultrafiltration membranes (molecular weight cutoff 100–500 kDa) are finding increasing applications for virus filtration. Validation of virus cleaner is a major concern in the biopharmaceutical industry. At the same time, purification of virus particles for viral vaccines and applications in gene therapy is a major separations challenge. Consequently, the second part of this chapter focuses on these membrane filtration applications.