As the market for low-pressure membranes expands, there is an increasing need for developing tools for assessing the performance of membranes. Several bench-test based tools for assessing membrane performance are discussed. Membrane Performance Index (MPI), a simple bench-test, can be used to evaluate the removal efficiency for targeted contaminants (e.g., TOC) and fouling potential of particular water source, or to assess the effectiveness of pretreatment options. Conceptually, MPI represents the condition where the rate of resistance to permeation is a first-order function. The validity of the test is based on the empirical correlations of the test results and actual data from a database consisting of pilot or production plants. Examples are used to illustrate how MPI is used to optimize the conditions of pretreatment to mitigate the impact of membrane fouling. For plant operations, membrane fouling is a major issue for the operation of membrane plants. Assessing the nature of fouling and optimizing the chemical cleaning procedure is critical to maintaining plant productivity while minimizing the plant downtime and operating costs. The procedures for a membrane cleaning study have been developed to address this issue. Examples applying this technique to diagnose the operational problems in the membrane pilot units and production plants are presented. Includes 12 references, table, figures.