Polyelectrolyte complexes are ionic hydrogels formed by the coreaction of two oppositely charged, strongly ionized polyelectrolytes. While insoluble in simple solvents and infusible, these polymers can be fabricated into membranes, fibers, and other shaped articles by casting from ternary solvents. Homogeneous membranes are optically clear, amorphous structures in which the equilibrium gel water content can be controllably varied from ca. 30-90%. The net ionexchange capacity of the membrane can be varied from zero to ca. 2.0 meq/g, either cationic or anionic. Homogeneous membranes are characterized by unusually high water and gas permeabilities and permeabilities to water and soluble solutes which can be controlled by adjustment of gel water content and ion-exchange capacity. Anisotropic membranes can also be prepared which are high flux molecular ultrafilters for solutes as small as sucrose. The anisotropic membranes have found widespread use in biology for fractionation, concentration, and purification of biopolymers. Homogeneous membranes show promise as analogues for biological membranes. A number of applications are being evaluated in medicine and surgery. These include: vascular grafts, antithrombogenic coatings for plastic prostheses, contact lenses, corneal implants, and surgical adhesives. I n addition, polyelectrolyte complex membranes are showing considerable promise for artificial kidneys and blood oxygenators.