Process Analytical Technology (PAT) is a manufacturing methodology for high value chemicals and pharmaceuticals. Critical parameters (CPPs) and key performance indicators (KPIs) of the process are thoroughly understood, well-defined and continually monitored in order to ensure that the pre-defined critical quality attributes (CQA) of the final product are consistently achieved. PAT measures key quality and performance indicators in raw materials, in-process materials and processes in real-time. A well designed PAT-based process is stable, ensuring that the critical parameters and indicators remain within pre-described limits to ensure product quality and process safety. PAT is an important element in a Quality by Design (QbD) system, in which quality is not tested into product, but rather inherent, by-design. PAT is an orthogonalized application of chemical, physical, microbiological, mathematical and risk analysis elements resulting in a complete understanding and control of processes. PAT is applicable to many standard operations performed in chemical processes, including reaction monitoring and crystallization procedures.