Removal or addition of dissolved gases from or to water or aqueous solutions can be accomplished very effectively using hollow fiber membrane contactors. Well-designed contactors, where water flow outside of the hollow fibers is directed transversely across banks of hollow fibers, exhibit significantly enhanced mass transfer coefficients and low pressure drop. The principle of operation, the features of the 'transverse-flow' design, and the mass transfer issues in such devices are discussed. The performance of membrane contactors and systems in degassing applications is presented. The predictability and the relative ease of scale-up of such membrane products are highlighted.