The C-H…O interaction, though weak, is not van deer Wails in nature but electrostatic and resembles O-H…O and N-H…O hydrogen bonds in its geometrical properties. Spectroscopic evidence hints at some degree of covalent bonding between H and 0 atoms. The long-range, electrostatic character of the C-H…O interaction determines its important role in crystal engineering. Planar aromatic hydrocarbons adopt herringbone structures but if a critical number of O atoms is present, a planar molecule will cross the structural threshold from a C…H stabilised herringbone structure to a C-H…O stabilised layer structure wherein adjacent molecules are parallel and highly overlapped. These ideas have been used to design a family of crystal structures of alkoxyphenylpro-piolic acids that participate in an intermolecular, solid state Diels-Alder reaction. When the number of C-H…O interactions in a structure is unusually large or small, there is a possibility of distortions in the O-H…O network.