This introductory chapter discusses what a river is; why rivers are important, both as natural features that shape the Earth’s surface and as resources for society; the science of fluvial geomorphology, including a brief history of this field; the process-oriented perspective of the book; rivers as dynamic natural systems; general concepts, such as dynamic equilibrium, thresholds, and nonlinear system dynamics, that have been employed to describe the dynamics of river systems; and the importance of both physical processes and environmental/historical contingency as factors influencing the dynamics of river systems. It presents a conceptual model defining rivers as dynamic systems characterized by interaction among flow, sediment transport, and channel form. It defines a hierarchy of spatial and temporal scales relevant to the characterization of river systems.