Membrane technology is a very emerging industrial process offering significant advantages in terms of low energy requirements, capital investments, easy to operate and compact equipment, in comparison with the traditional processes. This is due to the selective barrier, through the thin layer structure, which is called membrane, in which the separation occurs with relative lower energy demands. To this end, many types of materials, commercially available or synthesized in the laboratory, have been tested as candidate materials in various membrane applications. Properties such as, plasticization, solubility in inorganic or organic solvents, high thermal properties, flexibility, mechanical strength are required for the selection of the optimum material for every industrial application. In any case, the major factors that define the selection of a material as well as the efficiency of a certain filtration process are the high permeation and separation coefficients. The separation mechanism in a polymeric membrane mainly depends on material's nature combined with its formation process. Due to extreme differences between the polymer candidates many studies have been reported in the literature regarding the optimum materials for the different membrane types. Polyimides are a very attractive class of polymers that can be used on membrane technology, exhibiting high selectivities combined with high permeances for a large variety of applications including gas and liquid separations. In this article the principle theory of the mass transfer phenomena through polymers and a brief outline of polyimides based gas separation membranes are. presented and discussed. In specific, selected works of the preparation routine and the gas permeation/selectivity properties of polyimide based membranes are presented and discussed by giving emphasis on the carbon dioxide permeation and separation properties.