Functional Lanthanide-doped upconversion luminescent nanoparticles (UCNPs) are promising materials for theranostics due to their unexpected optical and chemical properties.UCNPs absorb low energy near-infrared (NIR) light and emit high-energy shorter wavelength photons.Their special features allow them to overcome various problems associated with conventional imaging probes.In this talk, I will present our recent progress in the development of UCNPs towards both single molecule study [1] and population-averaged in vitro and in vivo imaging [2].Engineering UCNPs towards photouncaging and therapeutic stem cell labeling [3] will also be discussed.