Crystallization has been the most important separation and purification process in the pharmaceutical industry throughout its history. Many parallels exist in the fine chemicals industry also. Over the past several decades the study of crystallization operations has taken on increasing levels of importance because of several factors that require effective control of crystallization processes. These levels of control require better understanding of the complex interactions of nucleation and growth as well as the operating characteristics of crystallization equipment including the critical issue of scale-up. In the pharmaceutical industry, the issue of better control, desirable in and of itself, is reinforced by the need to satisfy both company internal and governmental regulatory authorities on the consistency of chemical and physical properties of active pharmaceutical ingredients (APIs). Control of crystallization operations and choice of equipment both for pilot plant and manufacturing are thereby critical. The objective of this paper is to summarize the critical issues that must be addressed in order to achieve this level of control. (c) 2004 Elsevier B.V. All rights reserved.