The growth of crystals from a protein solution requires the existence of a phase transition, which allows the protein state to be manipulated between at least two thermodynamic phases: soluble and crystalline. Crystal nucleation and growth arise on the boundary between these two phases and are governed by subtle physico-chemical effects. The different zones of a two-dimensional phase diagram are illustrated schematically for the case of a Uox complex that has direct solubility (protein solubility increases with temperature)