Nanocomposites play an important role in the advancement of modern biomedical diagnostics. These multifunctional materials exhibit enhanced physicochemical properties, such as a high surface area-to-volume ratio, tunable optical and electrical characteristics, and excellent biocompatibility, which make them highly suitable for various biological applications. They have demonstrated remarkable sensitivity and specificity in detecting cancer biomarkers, infectious disease antigens, and metabolic enzymes. Beyond diagnostics, nanocomposites are increasingly used in theranostics, where they integrate diagnostic imaging with targeted therapy. This is particularly impactful in oncology, where metal and carbon-based nanocomposites enable dual functionality for tumor imaging and localized drug delivery. This chapter explores the major categories of nanocomposites employed in medical diagnostics and highlights their emerging applications in nanotheranostics, advancing the development of next-generation tools for precision medicine.