Currently, the most common form of drinking water treatment for surface water sources involves the chemical/physical removal of particulate matter by coagulation, flocculation, sedimentation, and filtration processes, along with disinfection to inactivate any remaining pathogenic microorganisms. Filtration remains the cornerstone of drinking water treatment, conventionally in the form of granular media depth filters. Although granular media filters can produce high quality water, they represent a probabilistic rather than an absolute barrier; consequently, pathogens can still pass through the filters and pose a health risk. The disinfection process provides an additional measure of public health protection by inactivating these microorganisms. However, some microorganisms, such as Cryptosporidium, are resistant to common primary disinfection practices such as chlorination and chloramination. Furthermore, drinking water regulations have established maximum contaminant levels (MCLs) for disinfection byproducts (DBPs) that may create incentive for drinking water utilities to minimize the application of some disinfectants. As a result of the concern over chlorine-resistant microorganisms and DBP formation, the drinking water industry is increasingly utilizing alternative treatment techlogies in an effort to balance the often-competing objectives of disinfection and DBP control. One such alternative techlogy that has gained broad acceptance is membrane filtration. Although the use of membrane processes has increased rapidly in recent years, the application of membranes for water treatment extends back several decades. Reverse osmosis (RO) membranes have been used for the desalination of water since the 1960s, with more widespread use of nafiltration (NF) for softening and the removal of total organic carbon (TOC) dating to the late 1980s. However, the commercialization of backwashable hollow-fiber microfiltration (MF) and ultrafiltration (UF) membrane processes for the removal of particulate matter (i.e., turbidity and microorganisms) in the early 1990s has had the most profound impact on the use, acceptance, and regulation of all types of membrane processes for drinking water treatment. The purpose of this guidance manual is to provide technical information on the use of membrane filtration and application of the techlogy for compliance with the Long Term 2 Enhanced Surface Water Treatment Rule, which would require certain systems to provide additional treatment for Cryptosporidium.