There is an urgent need to control materials on the molecular level to make “materials on demand.” A strategy to develop such materials is in development in reticular chemistry, derived from the Latin translation “reticulum” as “having the form of a net.” The strategy links discrete building units (molecules and clusters) via bonds to make large and extended crystalline structures. Metal-organic frameworks (MOFs) are the most prominent class of materials in the realm of reticular chemistry. Such crystalline, extended structures are constructed by stitching together inorganic polynuclear clusters known as secondary building units (SBUs) and organic linkers via strong bonds.