Biological macromolecules follow the same thermodynamic rules as inorganic or organic small molecules concerning supersaturation, nucleation, and crystal growth (1). Nevertheless macromolecules present particularities, because the intramolecular interactions responsible of their tertiary structure, the intermolecular interactions involved in the crystal contacts, and the interactions necessary to solubilize them in a solvent are similar. Therefore these different interactions may become competitive with each other. In addition, the biological properties of biological macromolecules may be conserved although the physico-chemical properties, such as the net charge, may change depending on the crystallization conditions (pH, ionic strength, etc.). A charged biological macromolecule requires counterions to maintain the electroneutrality of the solution; therefore it should be considered as a protein (or nucleic acid) salt with its own physico-chemical properties, depending on the nature of the counterions. To crystallize a biological macromolecule, its solution must have reached supersaturation which is the driving force for crystal growth. The understanding of the influence of the crystallization parameters on protein solubility of model proteins is necessary to guide the preparation of crystals of new proteins and their manipulation. Only the practical issues are developed in this chapter, and the reader should refer to recent reviews (2-4) for a description of the fundamental physical chemistry underlying crystallogenesis. The solubilization of a solute (e.g. a biological macromolecule) in an efficient solvent requires solvent-solute interactions, which must be similar to the solvent-solvent interactions and to the solute-solute interactions of the compound to be dissolved. All of the compounds of a protein solution (protein, water, buffer, crystallizing agents, and others) interact with each other via various, often weak, types of interactions: monopole-monopole, monopole-dipole, dipole-dipole, Van der Waals hydrophobic interactions, and hydrogen bonds. Solubility is defined as the amount of solute dissolved in a solution in equilibrium with its crystal form at a given temperature. For example, crystalline ammonium sulfate dissolves at 25°C until its concentration reaches 4.1 moles per litre of water, the excess remaining non-dissolved. More salt can be dissolved when raising the temperature, but if the temperature is brought back to 25°C, the solution becomes supersaturated, and the excess of salt crystallizes until its concentration reaches again its solubility value at 25°C (4.1 moles per litre of water).