Membrane contactors are a new way to accomplish separation processes like gas absorption and liquid-liquid extraction. They promise dramatically improved performance over conventional dispersed-phase contactors. They typically reduce the volume of equipment required for gas absorption by more than twenty times, and the volume of equipment for liquid extraction by more than five hundred times. This chapter describes what these contactors are and how they work. The practical issues to be considered in contactor design, and the advantages and disadvantages of membrane contactors are discussed. The easiest way to understand membrane contactors is by means of the specific example of antibiotic extraction. Conventionally, antibiotics are produced by fermentation. The fermentation beer, which is the feed, originally contains the antibiotic and the microorganisms. The beer is filtered to remove the microorganisms; the filtrate is then extracted with an organic solvent, frequently a higher alcohol, and an alkyl acetate or methylene chloride. The organic extract contains more concentrated antibiotic. This valuable solute is often further purified by additional extraction, by adsorption and by crystallization. The extractions can often be the most expensive step in this chain.