ConspectusDeep-ultraviolet (deep-UV, λ < 200 nm) coherent light is emerging as an indispensable driving force behind the innovation of optics and materials science. The deep-UV-driven applications range from laser interference photolithography to precise micromachining to futuristic ideas such as space propulsion using remotely controlled positioning lasers. Unlike conventional approaches to obtaining deep-UV light, for instance, synchrotron radiation, direct laser excitation, and gas discharge, nonlinear frequency conversion can be regarded as a more attractive way to endow such resource with high photon energy, high photon flux, and high spectral resolution. Actually, the nonlinear frequency conversion can be efficient only with the use of high-performing frequency-doubling crystals, which should be well-suited to the physics of nonlinear optical process. However, the necessary prerequisites for a practical frequency-doubling crystal are extremely strict, and thus very few crystals can be used to generate the deep-UV light. Faced with this, sustained effort has been expended by chemists and materials scientists toward discovering novel deep-UV frequency-doubling crystals. Studies have so far indicated that the main difficulty in finding a perfect candidate comes from the combination of three critical properties (absorption edge, nonlinear optical coefficients, and birefringence) into one crystal because they share the mutual relation of restriction and influence.In this Account, we present recent progress in discovering emergent deep-UV frequency-doubling crystals with the discussion of our efforts to balance the three critical properties by introducing the covalent tetrahedra [MO4–nXn] (n = 1–3), in which M refers to central atoms such as B, P, Si, S, Al, Zn, and Be and X can be apical atoms such as F, Cl, Br, and N. By analyzing the influence of the covalent tetrahedra on optical properties, we came to the conclusion of how to use the oxidized tetrahedra to achieve the improvement of the absorption edge, nonl...