We present new experimental results on second-harmonic generation in highly conjugated organic molecules and molecular crystals recently developed in our group, as well as on electro-optic effects in thin-film polymers. A large value of the molecular hyperpolarizability of a new hydrazone-type molecule crystallizing in a noncentrosymmetric point group is reported. Large nonlinear optical coefficients of up to 840 +/- 200 pm/V were measured in crystals of 4'-dimethylamino-N-methyl-4-stiloazolium tosylate . The new experimental results are compared with theoretical calculations of limiting values of nonlinear optical coefficients. Computer modeling of crystal structures shows good agreement with experimental results. Measurements on electro-optic polymers with high thermal stability are presented.