This chapter deals with the nature of soils and pays particular attention to those soil fractions that contribute directly to the assimilative capacity of soils. Only those properties of soils deemed pertinent in the various processes associated with transport of leachates in the soils are considered. The main items and issues considered in this chapter include: (a) nature of soil, such as origin, geologic and regional controls, soil fractions and compositional control on development of pertinent soil features, properties, and characteristics, (b) chemically reactive groups (defined as surface functional groups) on the surfaces of the various soil fractions making these surfaces reactive, thus allowing the particles to react chemically with solutes and other dissolved matter in the porewater, and (c) mechanisms associated directly with contaminant assimilation and attenuation (i.e., soil uptake) that can be optimized when all the available soil solids’ surfaces interact with the solutes in the porewater.