This review presents a comprehensive overview of the latest 20 years’ advancements in nonlinear optical crystals featuring stereochemically active lone pairs, with a focus on their crystal structures and optical properties, particularly their second-harmonic generation responses.

Nonlinear optical (NLO) crystals play a pivotal role in solid-state lasers and have broad applications in both civilian and military technologies. Ideal NLO crystal materials should possess a wide transmission range, moderate birefringence, a high second-harmonic generation (SHG) coefficient, and excellent physical and chemical stability. Recent research has highlighted that incorporating metals with stereochemically active lone pairs (SCALPs) can significantly improve the NLO performance of these materials. This review presents a comprehensive overview of the latest 20 years’ advancements in NLO crystals featuring SCALPs, with a focus on their crystal structures and optical properties, particularly their SHG responses. The article also explores how metal-based functional units containing SCALPs influence the polarizability of these materials. It is anticipated that this review will provide valuable insights into the functional design of high-performance NLO crystals, contributing to the discovery of new SHG materials with the desired properties to meet the growing demands of modern technologies.