A summary is presented on the mechanisms of dispersion of miscible contituents in the liquid phase of porous materials. The transport is considered from both a microscopic (within the pore spaces) and macroscopic (an average of a representative number of pores) point-of-view. An attempt has been made to point out some of the problems associated with the use of the physically measurable macroscopic variables when investigating the basically microscopic phenomenon of dispersion. Some simple reactions between soil materials and chemical substances are discussed and some applications to tracer studies are presented.