This chapter outlines models that provide a consistent explanation of many seemingly disparate observations. The models describe bulk behavior from a consideration of molecular behavior. They involve two principles of modeling: one is the principle of sufficiency, and the other is the principle of hierarchies. The term “variable charge” is used to describe those constituents whose charge varies with the pH of the soil solution. The charge is also affected by the amount of reaction with anions and cations that has occurred. Important variable-charge constituents of soils are the oxides of iron, aluminium, titanium, and manganese. The variable-charge constituents may be contrasted with the “fixed-charge” constituents, mostly the crystalline clay minerals that develop a negative charge as a result of lattice substitutions. The chapter determines that the surfaces with which ions react are large relative to the ions that they can be regarded as planes. The reactions of a diverse range of ions with soil are described by three rules. The first is that there is an initial reaction between a variable-charge surface and certain of the ions in solution—that is, reaction is between charged surfaces and charged particles. The second is that the soil surfaces are heterogeneous. And the third rule is that the initial surface reaction is followed by a diffusive penetration, the rate of which is modified by electrostatic effects.