Aluminum (Al) is the most abundant metallic element within the lithosphere. Although quantitatively important, the biogeochemical cycle of Al is complicated and poorly understood. Aqueous, particulate, and biological transformations may be conceptualized through a schematic representation of the Al cycle. Solution Al is the most chemically and biologically available form, although this pool represents a small fraction of the total Al in the environment. The mobilization and subsequent deposition of Al is an integral component in the development of well-drained soils. A variety of interrelated factors are critical to soil development and associated transport of Al, including climate, watershed elevation and slope, temperature, vegetation, and the nature of soil organic matter and minerals, as well as hydrologic flow paths. Streams generally exhibit the highest concentrations of Al of all surface waters. Stream Al concentrations are elevated in high elevation watersheds within acid-sensitive regions that have been impacted by acidic deposition.