Near-Infrared II (NIR-II) imaging technology is transforming biomedical imaging and disease diagnosis through the use of advanced probes such as quantum dots, lanthanide nanoparticles, carbon nanotubes, and organic dyes. Operating within the 1000–1700 nm wavelength range, NIR-II imaging offers deeper tissue penetration, higher spatial resolution, reduced autofluorescence, and superior signal-to-noise ratios compared with conventional NIR-I imaging (700–900 nm). These advantages have enabled significant progress in cancer detection, image-guided surgery, stem cell tracking, inflammation imaging, targeted drug delivery, and tumor vascular monitoring, resulting in enhanced diagnostic accuracy, improved therapeutic precision, and greater potential for personalized medicine.