This chapter prefers "Size exclusion chromatography (SEC)" for the separation of proteins because the term "SEC" includes the separation principle of the technique. SEC, a separation technique based upon the molecular mass of proteins, has been used successfully to monitor and remove low levels of contaminating proteins and aggregates. The separation in SEC of a complex mixture of proteins is independent of the eluent used and does not involve any binding between the sample in the mobile phase and the porous stationary phase bead. SEC can be used at any stage of a purification schedule because the proteins are separated in their native conformation, and there is good recovery of biological activity because there is no interaction with the resin. An alternative to dialysis is performing SEC using resins with a small pore size, which totally exclude the proteins in the applied sample from the porous space within the resin beads.