A large number of non-linear organic materials have been investigated by second-harmonic generation (SHG) in powders. The materials studied include several urea derivatives, sterically perturbed anilines and pyridines, and substituted stilbenes. Several compounds have been found with both a very large second-order non-linearity and a high optical damage threshold. Many compounds, even though known to possess large molecular hyperpolarizabilities, showed no detectable SHG, indicating a centrosymmetric crystal point group. The results are interpreted in terms of bond configuration, charge transfer and crystal structure. The use of good non-linear organic materials in integrated non-linear optics is discussed. © 1981.