Book excerpt: This volume focuses attention on understanding the physical origin of NLO effects in crystals at the molecular level and in particular the relationship between the macroscopic NLO effect of the crystals and their microscopic structure. Additionally, on the basis of theoretical and experimental studies, structural rules are described that facilitate the search for new NLO crystals in borate series compounds. C.T. Chen is a professor at thechinese Academy of Science where he works in the Fujian Institute of Research on the Structure of Matter