We report on the synthesis and optical characterization of hydrazone derivatives developed for second-order nonlinear optics. Electric-field-induced second harmonic generation (EFISH) as well as semiempirical quantum-chemical computations have been carried out to investigate and explain the nonlinear optical properties of these molecules. We obtained values of up to 220 x 10(-40) m(4)/V for the molecular hyperpolarizability beta at lambda = 1907 nm. The investigation of a series of 39 hydrazone derivatives with the second harmonic generation powder test according to Kurtz and Ferry revealed that an extraordinarily large fraction of these hydrazones show efficient second-order nonlinear activity. Among them, 4-(dimethylamino)-benzaldehyde-4-nitrophenylhydrazone (DANPH) and 4-(1-azacycloheptyl)-benzaldehyde-4-nitrophenylhydrazone (ACNPH) were selected for more detailed investigations. Their crystalline structures are presented, and the role played by hydrogen bonds in their molecular packing is emphasized. Finally, preliminary linear and nonlinear optical data of DANPH crystals are reported. [References: 42]