Many products are synthesized in the liquid phase but finally sold in solid form. Often the crystallization is one of the final steps of a synthesis and allows for the separation of a product from the liquid phase in very high purity. Even enantiomers can be separated from each other in many cases by using fractionating crystallization. We monitor crystallization processes online in order to optimize operating parameters and to obtain reproducable crystals with well-defined particle properties. Depending on the task, either specific product properties can be optimized or the handling of the produced solids in downstream processing can be simplified. An example is the optimization of filtration and drying times in the context of a debottlenecking. Furthermore, problems with milling or drying can often be avoided with crystallization.