Crystallizations of pharmaceutical active ingredients, particularly those that posses multiple polymorphic forms, are among the most critical and least understood pharmaceutical manufacturing processes. Many process and product failures can be traced to a poor understanding and control of crystallization processes. The Food and Drug Administration's process analytical technology (PAT) initiative is a collaborative effort with industry to introduce new and efficient manufacturing technologies into the pharmaceutical industry. PAT's are systems for design, analysis, and control of manufacturing processes. They aim to assure high quality through timely measurements of critical quality and performance attributes of raw materials, in-process materials, and final products. Implementation of PAT involves scientifically based process design and optimization, appropriate sensor technologies, statistical and information tools (chemometrics), and feedback process control strategies working together to produce quality products. This review introduces the concept of PAT and discusses its application to crystallization processes through review of several case studies. A variety of in situ analytical methods combined with chemometric tools for analysis of multivariate process information provide a basis for future improvements in modeling, simulation, and control of crystallization processes.