The same chemical compound can order itself into different crystal structures. Such polymorphic crystals have different physical properties, like solubility, shape and color. Understanding and control of polymorphism is highly important in the pharmaceutical industry, because in a drug usually only one specific polymorph is allowed. But the selection of the proper polymorph is also important for the material properties of pigments and dyes as well as of the fat crystals in chocolate. We investigate the polymorphic behavior of several organic molecules as a function of crystal growth conditions, by varying solvent, temperature, supersaturation, additives and crystal size, preferentially using in-situ observation techniques. In this we use carefully designed experiments, which are interpreted by extended computer modeling. This research continues to be relevant for several companies with which we collaborate.