A primary reason for flux decline during the initial period of a membrane separation process is concentration polarization of solute at the membrane surface This can occur in conjunction with irreversible fouling of the membrane as well as reversible gel layer formation Experimental and mathematical studies have been performed by various groups to gain a better understanding of concentration polarization phenomena in ultrafiltration and reverse osmosis This article critically reviews published studies on concentration polarization in both systems It presents progress made in determination of, for example, critical or limiting flux, and recommends specific models such as surface renewal, and experimental methods such as laser-based refractometry, for quantification of the problem.