The biological effects of ionizing radiation cannot be fully explained by considering only absorbed dose. Radiation type, energy, and individual cellular response all influence the way in which physical dose is converted to biological damage inside a living system. In the context of nuclear accidents, occupational exposures, and personalized radiotherapy, knowing the physical dose is not enough, as biodosimetry serves as a complementary tool. It leverages biological endpoints like DNA double-strand breaks, chromosomal aberrations, and mitotic disruptions to more accurately assess radiation damage. This chapter explores the methodologies and applications of biodosimetric assays, including molecular assays like γ-H2AX foci and the FIESTA-DNA probe, which quantify DNA damage rapidly, as well as cytogenetic methods such as DCA, FISH, CBMN, and PCC, which assess chromosomal instability over varying time scales. Applications of biodosimetry are discussed across emergency response, clinical radiotherapy, and research settings. The integration of biodosimetry with genomics, artificial intelligence, and advanced imaging is transforming precision medicine by enabling patient-specific protocols and predictive modeling of radiation effects. Limitations, including assay throughput, standardization, and ethical considerations such as data privacy and healthcare equity, are also addressed. In summary, biodosimetry bridges the gap between physical dose and biological effects, offering essential insights into radiation response. As technological and computational advancements continue to evolve, biodosimetry is positioned to play an increasingly vital role in radiation protection, diagnostics, and personalized medical care.