Upconversion is an optical process that includes the conversion of lower-energy photons into higher-energy photons. It has been widely studied since mid-1960s and extensively applied in optical devices. Over the past period, high-quality rare earth-doped upconversion nanoparticles have been successfully synthesized with the prompt development of nanotechnology and are becoming more prominent in biological sciences. The synthesis methods are typically phase-based processes, such as thermal decomposition, hydrothermal reaction, and ionic liquids-based synthesis. Rare earth doped upconversion nanoparticles, which can convert long wavelength near infrared radiation into short wavelength visible radiation via a non-linear optical process, are emerging as a new class of fluorescent biolables. They can overcome some of the limitations of conventional biolables including organic dyes and quantum dots. This book highlights recent advances in the synthesis, surface modification, and biological applications of rare earth doped upconversion nanoparticles. It serves as a powerful instrument that explores cutting-edge research knowledge on the topic of upconverting nanosystems, while simultaneously providing the necessary fundamental background for nonspecialist readers. The various aspects of upconverting materials are approached both from a theoretical point of view, particularly upconverting phenomenon, and a practical one. By presenting synthetic strategies, functionalisation, production of core shell structures and nanocomposites, this book supplies PhD students, researchers and practitioners, and scientists with a wealth of ideas they can apply to different fields of research. Predominantly, we highlight the advances in functionalisation strategies that enable the broad utility of upconversion nanocrystals for multimodal imaging, cancer therapy, volumetric displays and photonics.