Crystal engineering is the design and synthesis of molecular solid-state structures with desired properties, based on an understanding and exploitation of intermolecular interactions. The two main strategies currently in use for crystal engineering are based on hydrogen bonding and coordination bonds.
Organic crystal engineering relies on non-covalent bonds to achieve the organization of molecules and ions in the solid state. Much of the initial work on purely organic systems focused on the use of hydrogen bonds, though with the more recent extension to inorganic systems, the coordination bond has also emerged as a powerful tool. Other intermolecular forces such as weak hydrogen bonds, π…π stacking interactions, halogen bonding and hydrophobic interactions have all been exploited in crystal engineering studies. The archetypal molecular crystal is structurally anisotropic. This anisotropy is probed with nanoindentation, which also provides an understanding into the mechanical properties of crystals.

Molecular assembly is at the heart of crystal engineering, and it typically involves an interaction between complementary hydrogen-bonding faces or a metal and a ligand. By analogy with the retrosynthetic approach to organic synthesis, Desiraju coined the term supramolecular synthon to describe building blocks that are common to many structures and hence can be used to order specific groups in the solid state. The Cambridge Structural Database (CSD) provides an excellent tool for assessing the efficiency of particular synthons. The synthon approach has been successfully applied in the synthesis of a large variety of crystals including binary and ternary co-crystals. The modularity of the synthon can be studied with infrared spectroscopy. Synthon theory may be used to create new solid forms of active pharmaceutical ingredients with advantageous properties and patentable results.