The two kinetic steps, nucleation and crystal growth dominate the production process of crystalline products. In industrial crystallization. crystal size distribution and mean crystal size as well as external habit and internal structure are important characteristics for further use of the crystals. With regard to product characteristics. nucleation as the first of the two kinetic steps usually has a strongly predetermining influence on the second step crystal growth. On the other hand, nucleation is strongly interrelated to the width of the metastable zone or the metastability of a system set to crystallize, respectively. In order to design products by crystallization processes it is essential to measure on- and in-line supersaturation and metastability. Only optimum nucleation points as Well as optimum growth rates throughout the process call ensure the desired product quality. In other words, optimum crystallization processes call only be accomplished if the metastable zone width and the actual operation point of the crystallizer within this zone is known and controlled during the entire process. This necessitates sensors and control strategies capable of serving that purpose. (C) 2002 Elsevier Science B.V. All rights reserved.