The study of fluid flow in metamorphic environments progressed over the last quadrennium mainly from a focus on the tenets of continuum mechanics, in which rock properties such as porosity and tortuosity that control flow are assumed to be homogeneous over a specified finite volume. Advances have been afforded by these principles not only from their direct application, but also from tests of their veracity. Assumptions inherent in the continuum approach are based on the scale of observation and, in conjunction with the advent of new analytical technologies, have given new impetus for examining mechanisms of material transfer between fluids and metamorphic rocks at smaller and smaller scales. Thus, even as continuum mechanics has laudably come to be widely utilized in the study of metamorphic fluid flow, the ``empirical adequacy'' [ Oreskes et al., 1994] of the methodology is being assessed through access to previously unattainable scales of observation.
These new developments revealed some unexpected results. Among them are indications that fluid movement during regional metamorphism has been, at least in some cases, punctuated, with individual episodes lasting as little as several tens of thousands of years. This observation is particularly important in view of the widespread practice of estimating quantities of fluid on a time-integrated basis. Conventional ideas about the direction of fluid flow in variable temperature fields, the extent of elemental mobility, the scale of hydrothermal circulation during metamorphism, and the depth of infiltration of surface fluids have all been challenged by studies completed in the past four years.