Soil is a mixture of geological parent material, living organisms, and the colloidal residue of their interaction. ¤e colloidal fraction is particularly important in imparting soil with its unique characteristics. ¤e nature of soil exposes a large surface area that gives rise to adsorption of water and chemicals, ion exchange, adhesion and capillarity, swelling and shrinking, and dispersion and ¥occulation. Soil colloids include inorganic (clay minerals) and organic (humus) components. ¤e nature and amount of these colloids vary in response to environmental stimuli and organic input. ¤eir variation is responsible for lateral di¦erentiation in soil properties. Soils are separated vertically into horizons that indicate di¦erences in the internal soil environment, which, in turn, determines the amount and character of soil colloid accumulation. ¤is chapter provides an overview of the way in which soil properties vary as soils develop under varying environmental stimuli.