Transport through membranes occurs through small openings in the membrane material. Although the sizes of these openings range widely (sometimes even within a single membrane), two extreme cases can be considered. At one extreme are membranes in which fluid moves predominantly by advection through distinct pores. Most of the contaminant rejection achieved by these porous membranes occurs at the membrane surface, although some contaminants might enter the pores and get trapped before they exit. Porous membranes typically remove particles and most colloids, and those with the smallest pores can remove large molecules as well.