This overview describes some of the main features of the use of zeolite membranes for separation applications. Four different types of separations are considered: separation of non-adsorbing compounds, of organic molecules, of permanent gases from vapors, and of water (or polar molecules) from organic (or non-polar) species. Several factors, such as the limiting pore size and pore size distribution, surface diffusion, capillary condensation, shape selectivity and molecular sieving, contribute to the separations observed. However, in most of the high selectivity separations reported in the literature, preferential adsorption is the dominant characteristic. In this case, the adsorption of one component in the mixture is stronger and this blocks or hinders the passage of the other components through the membrane pores.