 Molecular imaging is rapidly developing towards multimodality and simultaneous therapy using upconversion nanoparticles, UCNPs, which have advantages such as low toxicity, large-stoke shifts, high resistance to photobleaching and photochemical degradation. Near-infrared excitation minimizes autofluorescence, increases penetrating depth, and reduces harmfulness to cells compared with traditional ultraviolet excitation. Composites of UCNPs with biological target molecules have superior performance for multimodality imaging, drug delivery, and targeted therapy. This review explores advances in UCNP applications for tumor imaging and therapy, discusses challenges and concepts, and aims to improve their use in biomedical imaging and therapy in the future. This article was authored by Na Zhou, Jian and I, and Rong Yi.