Upconversion nanoparticles (UCNs) are an emerging class of luminescent nanomaterials, exhibiting many advantages over conventional fluorophores, such as high signal-to-noise ratio and superior photostability. The near-infrared excitation wavelengths of these particles offer additional advantages such as deep tissue penetration and low photodamage to biological samples. In the last 5 years, with the advances in nanoparticles synthesis and modification technology, much research has been performed to exploit UCNs' advantages and integrate them into various biological applications. This review focuses on the recent developments of UCNs as imaging, detection and therapeutic tools, highlighting the respective strategies adopted.