Selling a chemical product with the required purity is today not sufficient for a chemical company to fill the gap in a market. This is the case with drugs or agrochemicals which are generally solid particles and have to exhibit some biological properties mainly connected to their physical quality. Among all the techniques which can be used to confer such properties, crystallization has a special position because it is the step of the process where the solid phase appears. In this paper, we discuss and give examples of how to mix physico-chemistry and chemical engineering in order to develop the crystallization of a product with specific properties for industrial exploitation or properties in use.