Besides the total amount of Al in a biological or toxicological sample a knowledge of the chemical form of the element is also important. The different species may have very different properties, e.g. their charge might be essential in their membrane transport behavior, and it is known that not all chemical forms of Al(III) are equally toxic. Speciation means a more or less exact knowledge of the compositions of the different species, including their bonding modes or structure and charge. In the case of the formation of labile Al(III) complexes, a speciation description can be given by using various methods permitting a study of solution equilibria and/or solution structure. pH-potentiometry is widely used to determine the stoichiometries and stabilities of the complexes formed, while multinuclear NMR is applied to obtain information on the most likely binding modes of the species. In spite of considerable efforts the picture is still rather contradictory concerning the speciation of Al(III) in biological fluids (blood, serum, saliva, stomach and intestinal fluids) or in various tissues.