Yes / Properties of a matter are intrinsically dependent upon the internal arrangement of molecules in the solid state. Therefore, knowledge of 3-dimensional structure of the matter is prerequisite for structure-property correlations and design of functional materials. Over the past century, X-ray crystallography has evolved as a method of choice for accurate determination of molecular structure at atomic resolution. The structural information obtained from crystallographic analysis paved the way for rapid development in electronic devices, mineralogy, geosciences, materials science, pharmaceuticals, etc. Knowledge of the structural information of active pharmaceutical ingredients (APIs) is prerequisite for rational drug design and synthesis of new chemical entities for development as new medicines. Over the past two decades, X-ray crystallography has played a key role in the design of pharmaceutical cocrystals-crystalline solids containing an API and one or more of pharmaceutically acceptable coformers. These materials have proved promising for fine-tuning several important properties of APIs. This short review highlights the history of crystallography, early breakthroughs, and the role of crystallography in understanding physicochemical properties of pharmaceutical cocrystals. / S. Aitipamula gratefully acknowledges the financial support from the Institute of Chemical and Engineering Sciences of A*STAR (Agency for Science, Technology and Research), Singapore. V. R. Vangala thanks Royal Society of Chemistry for Researcher Mobility Grant (2015/17).

The studies on cocrystals and salts presented in the the chapters clearly bring out the influence of intermolecular interactions as the main evaluators of the cocrystal-salt regime. The observations made in Chapter 2 indicate that in case if the cocrystal formation is through hydrogen bonds the location of the proton decides the nature of the complex in the energy landscape. The observation that the coformer controls the topology of intermolecular space as demonstrated in Chapter 3 provides insights into the importance of directionality rather than strength of intermolecular interactions. Indeed halogen bonding in cocrystals gain importance in this context.