Upconversion nanoparticles (UCNPs) have received profound interest in recent years as promising agents for biological studies with several advantages over traditional down conversion uorophores. They are nonblinking, nonphotobleaching, and able to emit short-wavelength light after sequentially absorbing two or more photons of near-infrared (NIR) light. Thanks to the advancements in nanotechnology, the development of UCNPs entered a higher level with many approaches to push UCNPs into the next generation, which are small in size with high upconversion emission intensities and tunable absorption/emission properties. This chapter focuses mainly on the recent progress in engineering excitation and emission properties of UCNPs systems.