Recent Advances in Upconversion Nanoparticles for Therapeutics: From Fundamentals to Cutting‐Edge Applications

ABSTRACT Optically responsive composite materials hold significant promise for in vivo diagnostics and targeted therapies. Rare‐earth‐doped upconversion nanoparticles (UCNPs), renowned for their unique luminescence properties, large anti‐Stokes shift, excellent biocompatibility, and deep tissue penetration, have emerged as highly promising candidates for advanced phototherapy in biological systems. This review first explores the fundamental mechanisms of upconversion luminescence, as well as synthesis, surface modification, and design strategies to brighten upconversion. It then highlights recent advances and key applications of UCNPs in biological therapy, including upconversion‐mediated phototherapy, multimodal therapeutic approaches, and image‐guided therapy and surgery. Finally, it discusses the current challenges and opportunities in both fundamental research and clinical translation, providing theoretical insights and practical guidance to support the broader application of UCNPs in biological therapy and clinical medicine.