An understanding of sorption processes is an important key to describing pollutant fate in an aquatic system because sorption may alter significantly physical transport and chemical reactivity of pollutants. The sorption of uncharged organic to natural aquatic sorbents is dominated by 'hydrophobic interactions.' For composite particulates (i.e., sediments/soils), organic matter is the primary sorbing constituent. Sorption partition coefficients, indexed to organic carbon K(oc), are relatively invariant for natural sorbents. K(oc)'s can be estimated from other physical properties of pollutants (aqueous solubility or octanol/water partition coefficients). Hydrophilic contributions to sorption tend to occur with one or both of the following conditions: (1) high sorbate polarity; and (2) low organic carbon content of the sorbent, especially with coincident high clay content. Although a prior estimation techniques comparable to hydrophobic sorption are not presently available, hydrophilic contributions relative to K(oc) can be estimated based on chemical class and sorbent composition. Although sorption to sediment or soils is frequently viewed as a rapid process in environmental modeling, true sorption equilibrium may require weeks to months to achieve, with pollutant uptake and release kinetics highly dependent on molecular size, sorbent cohesive properties, and solids concentration.