A better understanding of membrane microstructure and its role in the mechanisms of different separation processes is important in the development of new and more complex types of membranes. This understanding is particularly significant in the case of inorganic membranes because the permeation and separation properties are determined by the porous properties of this microstructure, which in turn is controlled by the synthesis route. This review will cover three topics: first, the origin and development of porosity in inorganic materials; second, the general definitions and terminology used to describe porous media; and third, a brief description of several techniques that can provide pore-structure characteristics of membranes.A distinction will be made between two techniques: static and dynamic methods. Dynamic methods are based on membrane permeation characteristics and are used routinely to assess membrane performance. This review, however, will be restricted to static methods, which can provide details of porous microstructure directly. These include the following: stereology (microscopic techniques such as scanning electron microscopy [SEM]); intrusive methods (e.g., mercury porosimetry, physisorption of gases, calorimetrie methods, and nuclear magnetic resonance [NMR] analysis); and nonintrusive methods (e.g., radiation scattering with neutrons and x-rays, wave propagation, ion-beam analysis, and positron lifetime spectroscopy). Finally, the suitability of these techniques in the characterization of membrane pore structure will be discussed.Porous materials can be formed in several ways, although the following three are most important in the synthesis of membrane structures.