Membrane reactors have found utility in a broad range of applications including biochemical, chemical, environmental, and petrochemical systems. The variety of membrane separation processes, the novel characteristics of membrane structures, and the geometrical advantages offered by the membrane modules have been employed to enhance and assist reaction schemes to attain higher performance levels compared to conventional approaches. In these, membranes perform a wide variety of functions, often more than one function in a given context. An understanding of these various membrane functions will be quite useful in future development and commercialization of membrane reactors. This overview develops a functional perspective for membranes in a variety of reaction processes. Various functions of the membranes in a reactor can be categorized according to the essential role of the membranes. They can be employed to introduce/separate/purify reactant(s) and products, to provide the surface for reactions, to provide a structure for the reaction medium, or to retain specific catalysts. Within these broad contexts, the membranes can be catalytic/noncatalytic, polymeric/inorganic, and ionic/nonionic and have different physical/chemical structures and geometries. The functions of the membrane in a reaction can be enhanced or increased also by the use of multiple membrane-based schemes. This overview develops a perspective of each membrane function in a reactor to facilitate a better appreciation of their role in the improvement of overall process performance.