Recent studies confirm that C-H...O hydrogen bonds have many, if not all, of the spectroscopic and physical earmarks of “traditional” H-bonds. Only more recently have we come to understand that these contacts are operative not only in protein-protein recognition, but enzyme-substrate interactions (with implications for drug design), biological and nonbiological transition state stabilization and have, in some circumstances, a strength and geometric predictability to make them useful in the construction of materials and supramolecular assemblies. Forays into each of these areas have come largely due to advances in computational protocols and spectroscopic instrumentation. This review discusses the most recent findings related to the nature (i.e. strength, geometrical preferences, and spectroscopic features) and function (i.e. in supramolecular design and synthesis, conformational control, enantioselective synthesis, and biology) of C-H...O interactions in a variety of chemical and biological systems. An understanding of the key features of these interactions should promote their deployment in target areas like medicinal chemistry and materials science.