 Abstract up-conversion is a non-linear optical phenomenon involving the emission of high-energy photons from the absorption of multiple low-energy excitation photons. These particles have been used for a variety of applications including bio-imaging, biosensing, theranostics, and photochemical reactions due to their excellent physical and chemical properties. Recent advances in the synthesis, optimization, and application of these particles are summarized here. This article was authored by Xiaoweizhu, Jingzhang, Jinnyanglu, and others.