The term sorption is used to refer to the many types of adsorption processes and mechanisms that result in the partitioning of dissolved substances in the porewater of a soil onto the surfaces of the soil particles. The sorption properties of soils are particularly important since the type and the amount of substances sorbed by a soil will change the nature of the soil, and hence affect its functionality. It can be said that these are probably the most important environmental soil properties. This can have significant impact in geoenvironmental engineering projects, groundwater management, and agriculture. The discussions in this chapter will focus on the nature of the interactions and mechanisms involved in soil sorption. Since the types of interactions and mechanisms involved in soil sorption processes are dependent not only on the surface properties of the soil particles, but as much on the nature and surface properties of the substances involved, the discussions will pay attention to (a) the surface properties of the interacting parties (soil particles and substances), and (b) the mechanism leading to soil sorption.