Strategies for the optimization of molecular second-order nonlinearities have emerged over the past few years, but significant questions still remain. This review highlights recent conceptual advances in the field of nonlinear optical molecular materials, concentrating on design criteria for the molecules themselves. e.g.. the influence of resonance structures, bond length alternation, and reduced aromaticity. and the use of organometallic compounds and octupolar molecules. Factors affecting the properties of bulk materials composed of such molecules are also discussed.