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.