The concept of a homogeneous polymer membrane, in which randomly coiled macromolecules pack the solid phase uniformly leaving some unoccupied space as free volume, has been applied in the study of permeability and selectivity of solutes through water (solvent)-swollen polymer membranes. Using this treatment, it became clear that fundamental principles of permeation through polymer film, i.e., a solution and diffusion mechanism of permeation, is also applicable to these swollen systems. However, the permeability characteristics of highly swollen systems resemble that of porous media due to the presence of the solvent, and the permeability of a swollen system to a solute cannot be characterized by a single permeability coefficient. The permeabilities based on diffusive flux (molecular flow) and on bulk flow of solvent, both of which occur under certain conditions, should be differentiated. Consequently permeability and permselectivity depend on the type of processes or conditions under which permeability is measured. Conformational entropies of macromolecules seem to play a predominantly important role in permselectivity and also in semipermeability (permselectivity between solute and solvent) of water-swollen membranes. Semipermeability depends on the combination of solute and solvent as well as on membrane characteristics such as swelling ratio. Membrane charge was found to have marked influence on permselectivities of ionized solutes. In general, the permselectivity decreases with increasing swelling ratio and consequent increasing permeability or flux of the solutes.