The research towards more efficient organic molecules for second-order nonlinear optical (NLO) applications has resulted in substantial improvements of the molecular nonlinear polarizabilities. Different strategies for increasing NLO responses at the molecular level in ionic chromophores are reviewed. However, only a small subset of the highly efficient non-centrosymmetric molecules also forms non-centrosymmetric crystals at the bulk level. Examples of success in achieving polar crystals of molecular salts are presented. Such crystals with a high number density of aligned dipolar chromophores are promising materials for efficient second-order NLO applications, such as terahertz generation via difference frequency mixing.