The present paper deals with the traditional and some new applications of the single crystal x-ray diffraction analysis of NLO materials. Traditionally, x-ray diffraction is used to prove molecular structure of a compound of interest, to establish crystal space group, packing array and features of the molecular geometry. This approach has been used in analysis of a large series of new organic NLO chromophores including substituted dicyanovinylaromatics, and some other NLO materials. Most of the compounds studied were found to demonstrate high molecular second-order optical susceptibilities. It was shown for the series of substituted dicyanovinylbenzenes what factors are responsible for the centric or acentric crystal structure of a given compound. Several new compounds of the series studied were found to exhibit a rather strong second harmonic generation signal in the solid state, in particular, 0-fluoro-dicyanovinylbenzene, p-dimethylamino- dicyanovinylbenzene, and 4-(4-methoxyphenyl)-1,1-dicyano- 1,3-butadiene, 4-MeO-C6H4-CHequalsCH-CHequalsC(CN)2. Molecular and crystal structures of these compounds have been studied and analyzed. Another new application of the x- ray diffraction method in the study of NLO compounds is analysis of the electron density distributions in crystal and direct estimation of some of its characteristics responsible for NLO properties directly from the diffraction data. These opportunities of the method were demonstrated in the charge density study of crystals of DIVA and nMA. Second order optical susceptibilities were estimated from the diffraction data using a multiple model and were found to be close to the experimental values.