This handbook emphasizes the use of synthetic membranes for separations involving industrial or municipal process streams. Little will be said concerning the use of membranes in medical applications as in artificial kidneys or for controlled drug release. Most of the membrane processes are pressure driven. The notable exception to this is electrodialysis (ED) by which ions are separated under the influence of an electric field. In addition, the chapter on coupled transport covers processes which are driven under the influence of a concentration gradient. Pressure driven processes include microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), pervaporation (PV), and gas separations (GS). With the exception of pervaporation, which is just beginning to emerge as an industrial process, each of the above will be covered in a separate chapter. The pressure driven liquid filtration processes (MF, UF, and RO) may be distinguished by the size of the particle or molecule the membrane is capable of retaining or passing. This roughly relates to the pore-size of the membrane. Figure P.l shows the pore sizes of MF, UF and RO membranes. Obviously, all particles or molecules larger than the rated pore size will be retained. The smallest particle which can be seen with the naked eye, under the best of lighting conditions, is about forty (40) microns in diameter. A typical human hair has a diameter of eighty (80) microns. This means that membranes are filtering particles out of solution which are invisible to the naked eye. Even the most open MF membrane is capable of retaining yeast (3 to 12 microns) and tighter MF membranes can retain the smallest bacteria (Pseudomonas diminuta, 0.2 microns) (see Figure P.2). The dividing line between MF and UF membranes at 0.1 microns (1000 8) is somewhat arbitrary. The most open UF membranes (almost always anisotropic) have a molecular weight cut-off of approximately one million (106) daltons which corresponds to about 0.08 microns. There is some overlap in pore size because there are MF membranes (usually isotropic) available with pore sizes down to 0.02 microns. However, these MF membranes are used only for analytical applications and have no commercial importance for large scale processing.