The basic processes underlying partially saturated flow and, more generally, multiphase flow in porous media are explained in a synthetic way. The definitions, assumptions, and conventions generally considered when dealing with partially saturated flow are proposed and the main differences among saturated conditions are noted. Emphasis is given to the important variations (i.e., nonlinear behavior) of the parameters (e.g., the hydraulic conductivity) as a function of the water content. The basic partially saturated flow equations are expressed with the water pressure or hydraulic head as the main variable. Then, equations of solute transport in the partially saturated zone or vadose zone are written with a focus on transport specificities resulting from the partial saturation of the medium.