Under the proposed Long-term 2 Enhanced Surface Water Treatment Rule, water suppliers using membranes must conduct periodic direct integrity testing and continuous indirect monitoring to receive credits for Cryptosporidium removal. Membrane filtration facilities currently use pressure hold tests to detect minor breaches of membrane integrity and particle counting and/or turbidity monitoring to meet regulatory requirements and detect more-pronounced breaches in membrane integrity. However, no single test for monitoring membrane integrity satisfactorily fulfills the criteria for all requirements. This article provides an overview of direct and indirect membrane monitoring techniques, summarizes their strengths and weaknesses, and identifies current trends, future developments in membrane integrity monitoring, and areas where further research is needed. In lieu of a single "ideal" technique for monitoring membrane integrity, the authors propose a comprehensive integrity monitoring program that incorporates several existing methods. Includes 56 references, tables, figures.