The growing world of crystal forms.

Engineering crystal properties by a choice of adequate molecular building blocks has to cope with the fact that a given molecule can be isolated in many, at times very many, solid forms (single entity crystals, solvates, salts, co-crystals and their polymorphs) each with its own characteristics and properties. Preparation of new crystal forms of a same molecule, and investigation of the relationship among new crystal forms represent not only a further challenge to the solid state chemists, but also an opportunity of modifying properties of crystals with applicative implications (pharmaceuticals, agrochemicals, pigments, organic semiconductors, etc.). This feature article elaborates on this concept via a number of examples of multiple crystal forms and test cases.