Fluid flow, as it applies to geologic media, is the topic of this volume. The range of interest is large; it encompasses the weathering of geologic formations by the action of water, the manner in which certain minerals come to occur in commercial quantities, the fate of chemical contaminants once they enter an aquifer, optimal methods to remove (or at least contain) these contaminants, and ways to improve the recovery of hydrocarbons from reservoirs. While it is impossible to treat all of the applications of geochemical flow in a single volume, it is possible to treat certain features of simplified reactive flow that occur in nearly all applications. Understanding these features will help interpret much more complex flows and providing the basis for this understanding is the goal of this text. This book is a culmination of a research project conducted at The University of Texas at Austin (UT) over the past 20 years. It has also been used as a text in a graduate course at UT on geochemistry and flow, taught by each of the editors over a period of 10 years. The reader will undoubtedly benefit from the knowledge flow that this progression from research project, via classroom, to text represents.