Oxidation-reduction (redox) reactions are among the most important and interesting chemical reactions that occur in aquatic environmental systems, including soils, sediments, aquifers, rivers, lakes, and water treatment systems. Redox reactions are central to major element cycling, to many sorption processes, to trace element mobility and toxicity, to most remediation schemes, and to life itself. Over the past 20 years, a great deal of research has been done in pursuit of process-level understanding aquatic redox chemistry, but the field is only beginning to converge around a unified body of knowledge. This chapter provides a very broad overview of the state of this convergence, including clarification of key terminology, some relatively novel examples of core thermodynamic concepts (involving redox ladders and Eh-pH diagrams), and some historical perspective on the persistent challenges of how to characterize redox intensity and capacity of real, complex, environmental materials. Finally, the chapter attempts to encourage further convergence among the many facets of aquatic redox chemistry by briefly reviewing major themes in this volume and several past volumes that overlap partially with this scope.