Ultra filtration (UF) is a membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane. Ultrafiltration is not fundamentally different from reverse osmosis, microfiltration or nanofiltration, except in terms of the size of the molecules it retains.

UF's main attraction is its ability to purify, separate, and concentrate target macromolecules in continuous systems. UF does this by pressurizing the solution flow. The solvent and other dissolved components that pass through the membrane are known as permeate. The components that do not pass through are known as retentate. Depending on the Molecular Weight Cut Off (MWCO) of the membrane used, macromolecules may be purified, separated, or concentrated in either fraction.