 Abstract upconversion is a nonlinear optical phenomenon involving the emission of high-energy photons from the absorption of multiple low-energy excitation photons. This process has been widely used for biomedical imaging, sensing, therapeutics, and photochemistry due to its deep penetration depth, minimal damage to samples, and high chemical stability. Recent advances in the synthesis, optimization, and application of upconversion nanoparticle, UCNP, based materials have been reviewed. Strategies for optimizing UCNP emissivity and applications of UCNPs have been discussed, along with potential challenges and future directions. This article was authored by Xia Weizhu, Jing Zhang, Jin Lianglu, and others.