Trace metal speciation is often studied by applying an analytical methodology. An alternative method is based on the prediction of the distribution of chemical species by using computer modeling techniques. This approach involves solving multiple simultaneous equations, some of which describe the competing chemical equilibria and others mass balance relationships, and requires the input of data on the total level of cations, anions, and ligand species present and equilibrium constant values for all possible interactions between the various species. The approach is not new. However, today the technique is potentially more readily available because of advances in computer technology, and speciation modeling can now be considered a routine tool in many fields. In this chapter, the main aspects of thermodynamic speciation modeling are described. The reader progresses from fundamental concepts, such as the idea of chemical component or the presentation of the well established adsorption models used to describe trace metal sorption on solid phases, to the discussion of the limitations imposed by the need of additional and reliable thermodynamic data and the difficulties involved in obtaining a rigorous thermodynamic description of the complexing properties of natural organic matter.