The article presents the principles of liquid-liquid extraction in aqueous two-phase systems which offer a great potential for the recovery and purification of various biomolecules, cells and organelles from fermentation slurries. These systems consisting of a solution of two polymers (e.g. polyethylene glycol/dextran), polymer and salt (e.g. polyethylene glycol/phosphate) or a thermoseparating polymer (oxyethylene-oxypropylene copolymer) offer mild separation conditions due to the high concentration of water, which makes them suitable for biotechnology applications.