in challenging tasks including single-molecule tracking and deep tissue imaging. In the case of bioimaging, as lanthanide-doped nanoparticles can be excited with near-infrared light, they can reduce autofluorescence of biological samples and thus improve the contrast of the image. Lanthanide-doped nanoparticles are nanocrystals of a transparent material (more often the fluorides NaYF4, NaGdF4, LiYF4, YF3, CaF2 or oxides such as Gd2O3) doped with lanthanide ions. The most common lanthanide ions used in photon upconversion are the pairs erbium-ytterbium (Er3+,Yb3+) or thulium-ytterbium (Tm3+, Yb3+). In such combinations ytterbium ions are added as antennas, to absorb light at around 980 nm and transfer it to the upconverter ion. If this ion is erbium, then a characteristic green and red emission is observed, while when the upconverter ion is thulium, the emission includes near-ultraviolet, blue and red light. Despite the promising aspects of these nanomaterials, one urgent task that confronts materials chemists lies in the synthesis of nanoparticles with tunable emission, which is essential for applications in multiplexed imaging and sensing. The development of a reproducible, high yield synthetic route that allows controlled growth of rare earth halide nanoparticles has enabled the development and commercialization of upconversion nanoparticles in many different bioapplications. The recent progress in this direction includes the synthesis of structured nanocrystals crystals, such as particles with a core/shell structure, allowing upconversion through interfacial energy transfer (IET). Semiconductor nanoparticles Semiconductor nanoparticles or quantum dots have often been demonstrated to emit light of shorter wavelength than the excitation following a two-photon absorption mechanism, not photon upconversion. However, recently the use of semiconductor nanoparticles, such as CdSe, PbS and PbSe as sensitizers combined with molecular emitters has been shown as a new strategy for photon upconversion through triplet-triplet annihilation. They have been used to upconvert 980 nm infrared light to 600 nm visible light; green light to blue light; and blue light to ultraviolet. This technique benefits from a very high upconverting capability. Especially, these materials can be used to capture the infrared region of sunlight to electricity and enhance the efficiency of photovoltaic solar cells. Upconversion nanocapsules for differential cancer bioimaging in vivo Early diagnosis of tumor malignancy is crucial for timely cancer treatment aimed at imparting desired clinical outcomes. The traditional fluorescence-based imaging is unfortunately faced with challenges such as low tissue penetration and background autofluorescence. Upconversion (UC)-based bioimaging can overcome these limitations as their excitation occurs at lower frequencies and