The paper reviews and anlyses the most widespread up-to-date designs of membranous oxygenators. On an example of some types of membraneous oxygenators and their functional characteristics basic parameters for a comparative evaluation and choice of the "membranous lung" and its effective utilization have been deduced, viz. the circulation rate, the rate of the oxygen and carbon dioxide transport (per 1 m2 of the oxygenating surface), pressure differential, primarily filled up volume and oxygen consumption. The authors underline the need for determining the efficiency of the oxygenator with reference to O2 and CO2 and standard values of unsaturated blood indices. With due regard for promising trends in the future development of artificial "membranous lungs" features that may be of use in the classification of membraneous oxygenators have been systematized.