The relevance of co‐crystals for pharmaceutical development is demonstrated by the constant growth of the number of papers and patents published in the two last decades. Co‐crystallization appears to be an efficient means to modulate the solid state properties of a pharmaceutical active ingredient (API) within the crystalline state. It is also a crystal engineering approach enabling to prepare original materials, including co‐drugs. Co‐crystallization is therefore a route to innovation and opens perspectives to patent protection. Alternatively, co‐crystal formation may represent an undesired or unexpected outcome of molecular complexation, for example in the drug product. This observation underlines the importance of a detailed study on the solid state chemistry not only of the pure API, but also of its combinations with the individual components in the formulation.