This chapter discusses the determination of aluminum in natural waters, with an emphasis on freshwater systems. The definition of natural fresh waters, as used in this chapter, includes lakes and streams, as well as groundwaters and interstitial water in soils and sediments. Determination of aqueous aluminum is important both to scientists interested in the toxicity of aluminum to terrestrial and aquatic organisms and to those who study mineral weathering and mineral neoformation. Measurement of aluminum emission in aqueous samples is accomplished generally by using an inductively coupled plasma as the excitation source, and our discussion of emission will be confined to plasma techniques. Determination of aluminum by reacting it with an organic reagent and measuring the light absorbed by the aluminum-organic complex is very common. The fluorescence of many aluminum-organic complexes forms the basis of several very sensitive analytical techniques.