Toxicity of aluminium depends on its chemical forms occurring in nature. Unequivocally toxic for live organisms are inorganic aqua/hydroxo monomeric and some polymeric complexes of aluminium. Most laboratory speciation methods use operational definitions of determined species, depending on the type of the applied method. The principles of the most frequently used laboratory methods are based on the size and shape of particles or molecules of Al compounds (filtration and ultrafiltration, dialysis, gel chromatography), on different kinetics of formation of stable complex Al compounds and on the type and bond strength in Al compounds (ion exchange and ion chromatography). In addition to a survey of methods of Al speciation in water and soil solutions, methods based on chemical equilibrium calculations used for identification of chemical forms of aluminium or for comparison with experimental results are given.