This chapter considers the development of drainage basins and stream networks over geological timescales. It examines why a focus on river systems over geological timescales is important, how river channels form through surface and subsurface processes, and the differences between stream channels and hillslope erosional features, such as rills and gullies. It explores how drainage networks are related to properties of drainage basins and how stream networks develop, grow, and evolve over time. It also examines how channels are initiated on the landscape and how drainage-basin evolution and channel network evolution are intertwined. The spatial geometry and arrangement of river networks is reviewed along with the scaling properties of these networks.