Fourteen percent of U.S. natural gas contains excess nitrogen, and cannot be sent to the national pipeline without treatment. Nitrogen is difficult to economically separate from methane, by any technology. Currently, the only process used on a large scale is cryogenic liquefaction and fractionation, but this technology requires economies of scale to be practical. Many owners of small gas fields cannot produce their gas for lack of suitable nitrogen separation technology. This paper describes the development of selective membranes to treat natural gas containing high concentrations of nitrogen. Membranes selectively permeate either nitrogen or methane, the principal constituent of natural gas. Our work has shown that methane-selective membranes are generally preferable and membranes with high permeances and methane/nitrogen selectivities of approximately 3-3.5 were developed. This selectivity is modest, so commercial systems often require multi-stage or multi-step process designs. Despite the design complexity and compression requirements, multi-step/multi-stage membrane systems are the lowest cost nitrogen removal technology in many applications. To date, 12 membrane-based systems for nitrogen removal for natural gas processing have been installed. (C) 2009 Elsevier B.V. All rights reserved.