This chapter provides an overview of the hydrolytic reactions of Al in aqueous solutions and presents the salient features of polynuclear Al structures. It evaluates current methods of detecting and quantifying polynuclear forms of Al and discusses the mechanisms of their formation and certain implications of the polynuclear Al species to soil-water systems. Recent studies have utilized spectrophotometers that permit continuous monitoring of absorbance increases that occur after rapid mixing of the ferron reagent and a partially neutralized Al solution. Th chapter also highlights the origins and genesis of polynuclear Al species. A potentially important mechanism for the formation of polynuclear Al species involves polynucleation at the mineral water interface. Nucleation of Al mononuclear and polynuclear species into colloidal or distinct solid phases is an important phenomenon utilized in physicochemical treatment of water and wastewaters.