Artificial membranes, with pore sizes smaller than a few nanometers, are widely studied for separation and reaction purposes. Particularly they make it possible to imagine original processes for the separation of small molecules or ions. The improvement of the molecular size cut-off could be interesting for the yield enhancement in continuous reactive process on specific fields where traditional techniques prove to be inoperative due to some intrinsic incapacity, excessive costs, non-observance of environmental constraints, etc. The structure of the membrane may be changed according to the type of the material from which it is built, and its use extended to the various states of fluids, i.e. gas, liquid or supercritical. In this paper a synopsis of the most recent developments on the subject is presented.