Until recently, a majority of studies used purely scalar properties (e.g., equilibrium thermodynamics) to describe metamorphic fluid flow. In the last quadrennium, however, there has been widespread, albeit belated, acknowledgement among metamorphic petrologists and geochemists that fluid flow, being an inherently vectorial phenomenon rather than a scalar phenomenon, must be examined using transport theory; accurate description of fluid-rock interactions requires use of field variables, variables which are functions of both position and time. Continuum mechanics provides a framework for relating both field and scalar-valued variables using volume-averaged properties of the porous rock system. Averaging is performed over some``representative elementary volume,'' or REV. The REV size for a particular system depends implicitly on the scale of observation but in any case must be significantly larger than the size of individual fluid pathways (i.e., pore size, grain size, or fracture width).