A model is developed that calculates capital and operating cost of cross-flow membrane filtration as a function of relevant design parameters. An expression for capital cost is empirically derived from information gathered from membrane manufacturers. Operating cost is divided into components for energy consumption, chemical demand, membrane replacement and concentrate disposal and is determined from attributes of the membrane system. The effect of operating parameters and membrane characteristics on cost are examined. The flux of permeate across a membrane is the principle factor determining cost. Energy cost, although important to the daily operating cost of a water treatment plant, is overshadowed by the membrane replacement cost. The effect of backflushing procedures on cost is evaluated using empirical functions for permeate flux derived from a microfiltration pilot study. Microfiltration is determined to be a cost-competitive alternative to ultrafiltration. Costs calculated by this study are compared to reported costs for membrane filtration.