This chapter reports an analysis of literature dedicated to the speciation of cadmium in various environmental compartments, i.e., atmosphere, natural waters, soils and sediments. The difficulty of the cadmium speciation studies, due to the variability of composition of different natural systems and to the low cadmium concentration in the environment, is highlighted. As an alternative approach, cadmium behavior is assessed by modelling its reactivity towards the main classes of ligands usually present in natural systems. The stability of cadmium complexes with various ligand classes is analyzed and modelled. Simple equations are proposed for the estimation of the stability of cadmium complexes with carboxylates, amines, amino acids, complexones, phosphates, phosphonates, and thiolates. The modelling ability of these equations is carefully analyzed. In addition, the sequestering ability of some ligands toward cadmium has been evaluated by the calculation of pL(0.5) (the total ligand concentration, as -log c (L), able to bind 50% of a metal cation), an empirical parameter recently proposed for an objective "quantification" of this ability in defined conditions (pH, ionic strength, temperature, composition of solution).