Ultrafiltration is a pressure-driven, membrane-based separation process, which is used for a broad variety of applications, ranging from the processing of biological macromolecules to wastewater treatment. It has significant advantages over competing separation technologies. Food and biotechnological applications account for nearly 40% of the current total usage of ultrafiltration membranes. Protein bioseparation is an important component of this application segment. Ultrafiltration is used for protein concentration, desalting, clarification and fractionation (i.e. protein–protein separation). Concentration, desalting and clarification are technologically less demanding and have been in used in the bioprocess industry for some time. Protein fractionation, on the other hand, is a challenging proposition and is definitely a more recent development. This book focuses primarily on protein fractionation.