Porous structures for membranes have been obtained by several methods, the most usual one being phase separation. An asymmetric porous structure is formed when a polymer solution is cast on a nonwoven substrate and immersed in a nonsolvent bath. Phase separation initiates by nucleation and growth, or by spinodal decomposition, yielding a polymer-rich matrix. The final structure is frozen by a mobility decrease due to gelation or solidification. Some alternative methods for membrane formation are also mentioned.56187192