Crystal engineering is the design and synthesis of molecular solid state structures with desired properties, based on an understanding and use of intermolecular interactions. The two main strategies currently in use for crystal engineering are based on hydrogen bonding and coordination bonding. These may be understood with key concepts such as the supramolecular synthon and the secondary building unit.
The term ‘crystal engineering’ was first used in 1971 by Gerhard Schmidt in connection with photodimerization reactions in crystalline cinnamic acids. Since this initial use, the meaning of the term has broadened considerably to include many aspects of solid state supramolecular chemistry. A useful modern definition is that provided by Gautam Desiraju, who in 1988 defined crystal engineering as "the understanding of intermolecular interactions in the context of crystal packing and the utilization of such understanding in the design of new solids with desired physical and chemical properties." Since many of the bulk properties of molecular materials are dictated by the manner in which the molecules are ordered in the solid state, it is clear that an ability to control this ordering would afford control over these properties.