Membranes are fabricated from a variety of starting materials,and they are implemented in various physical forms. Inthis section, we describe a few physical/chemical featuresof the membranes used most commonly in water-focusedenvironmental engineering applications and relate thosefeatures to membrane performance.Transport through membranes occurs through small openingsin the membrane material. Although the sizes of theseopenings range widely (sometimes even within a singlemembrane), two extreme cases can be considered. At oneextreme are membranes in which fluid moves predominantlyby advection through distinct pores. Most of the contaminantrejection achieved by these porous membranes occurs at themembrane surface, although some contaminants might enterthe pores and get trapped before they exit. Porous membranestypically remove particles and most colloids, andthose with the smallest pores can remove large molecules aswell.