An analysis is given of various aspects of the design of crystallization-based separation processes. A literature review is included for each of the different themes treated in the study. Special emphasis is placed on the usefulness of the relative composition diagram as a tool for identification of possible flow sheets for separations based on fractional crystallization. A series of rules are derived that may be of practical value in the use of the relative composition diagram. Illustrative examples are included that present the advantages and limitations of this methodology. Results are compared with those obtained from the literature where available. (c) 2006 American Institute of Chemical Engineers.