Precipitation and crystallization have long been common components of hydrometallurgical processes involving rare earth elements (REEs), for purposes of concentration, separation, and recovery. In this chapter, different possible crystallization-based techniques for separation of REEs as solid compounds from solution are reviewed. Some of the techniques are well-established, while others are promising alternatives of lower technological maturity. Fundamental theory common to all methods of crystallization of ionic compounds is covered, solubility data of different rare earth salts are presented, and important aspects of the underlying chemistry behind rare earth crystallization are highlighted, in order to rationalize the behavior of the different REEs and to give a deeper understanding of the phenomena at hand. The chapter supplies information helpful in the design of a suitable and efficient crystallization-based REE-separation process.