This review on polymorphism is a personal, non-comprehensive view on the field of polymorphism -a term which is often misused. Indeed, the discussion about polymorphism and related terms is still ongoing in the area of crystal engineering. This is why we felt it timely to look into the historical development of its definition and to delimit it. A short introduction to thermodynamic aspects and characterization methods of polymorphs is given. One chapter is then dedicated to polymorphism of elements and inorganic compounds, before discussing the term for organic and organo-metallic compounds. Chosen examples are given each time to illustrate the cases of polymorphism. In the end, the conclusion yields three flow schemes useful in determining polymorphism for each compound class.

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Polymorphism is a very important phenomenon not only in basic research, but certainly in pharmaceutical industry and materials science. Polymorphs possess different properties, for instance the solubility or the mechanical resistance can differ dramatically from one polymorph to the other – properties which can be crucial for their application. Hence, it is important to be able to control the formation of polymorphs and to understand their formation. We here gave some insights into the basic knowledge of polymorph formation and their identification and characterization in order to give an overview on the current state of the art. In order to give interested peoples a tool in hand to test their compounds for polymorphism, we established a series of flow sheets to follow, depending on the class of compounds, hoping that they are useful for many scientists who are not so well acquainted with polymorphism. The presented schemes resume thus the identification steps for polymorphs. It should also help to use the term polymorph correctly in order to reduce the number of publications in which this term is not used in a correct way.