This chapter emphasizes on the significance of aluminum-organic interactions in soils, the distribution of naturally occurring organic ligands that bind aluminum, and the nature and stability of aluminum-organic complexes. Although primary consideration will be given to terrestrial systems, many of the concepts discussed apply to aquatic environments as well. The chapter discusses the eluviation and transport of aluminum in soils and the Influence of organic substances in ameliorating aluminum toxicity. Humic substances may be of greater importance than biochemical compounds in modifying the reactivity of aluminum on oxide surfaces, reducing toxicities of aluminum in acid soils, and binding aluminum in most natural waters. The ability of humic substances to form stable complexes with polyvalent cations, including aluminum, is due to their high content of oxygen-containing functional groups. Humic and fulvic acids isolated from mineral soils invariably contain appreciable amounts of metal ions, including aluminum.