Transport phenomena, that are relevant to ultrafiltration membranes, are discussed. Their basic characteristics are given by the pore-flow theory. In actual applications, however, their behaviors are controlled by gel layer formation, osmotic pressure of solutes, adsorption or plugging in pores and by other chemical and physical natures of membranes used. Also electric charges give rise important effects both on flux and solute separation. All these behaviors need careful analysis of data taken by model and actual solutions. Also recently distinction between RO and UF membranes becomes obscure and their relation should be made more clear. These results will finally be combined to establish methods to select proper membrane materials and to design proper membrane modules for actual applications.