The quest for more efficient nonlinear optical materials of increased optical quality is being spurred by the development of optical communication systems that require ultrafast broadband optical signal processing functions. Conversely, nonlinear optical phenomena enlarge traditional spectroscopic approaches to encompass more complex and informative multiphotonic pathways. In this stimulating context, organic nonlinear materials have been recognized as forefront candidates for investigations of fundamental and applied natures involving in a joint effort chemists, material scientists, and optical physicists. After reviewing the molecular engineering foundations of the domain, specifically with respect to an efficiency-transparency trade-off, we concentrate on a more specific case that embodies, from the definition of the molecule, all the way down to applied physics end goals, the key-concepts and methodological tools in the field. This is the paradigmatic case of N-4-Nitrophenyl-(L)- Prolinol (NPP) which, after almost a decade of efforts involving crystal growth and ultrafast time resolved spectroscopy, has recently reached sufficient quality to lead to optical parametric oscillation. Stable coherent tunable oscillation in the near IR from 0.9 to 1.7 micrometers has thus been demonstrated for the first time in an organic molecular crystal with specific advantages as compared to mineral candidates and room for further performance improvements. We recall in conclusion that new molecular and crystalline pathways are still opening-up, such as based on molecular nonlinearities of octupolar origin or organo-mineral crystals.