Ionizing radiation plays a central role in oral and maxillofacial radiology, underpinning both diagnostic imaging and radiation-based therapeutic applications. This chapter provides a comprehensive, concept-driven exposition of radiation physics and biology as they relate to dental and maxillofacial practice. Fundamental physical concepts, including atomic structure, particulate and electromagnetic radiation, X-ray generation in the Coolidge tube, Bremsstrahlung and characteristic interactions, and the physical behavior of X-rays in matter, are explained in detail. Image-determining variables such as beam quality, quantity, filtration, collimation, inverse square law, focal spot geometry, and the physics of detector systems are contextualized to clinical imaging. Biological responses to radiation are discussed across molecular, cellular, and tissue levels, emphasizing direct and indirect DNA damage, free-radical chemistry, cellular radiosensitivity, deterministic and stochastic effects, and modifying factors such as linear energy transfer (LET), relative biological effectiveness (RBE), oxygen effect, dose-rate, and fractionation. This chapter integrates radiobiological principles with clinical scenarios relevant to dental radiology, including salivary gland dysfunction, mucositis, radiation caries, microbial dysbiosis, and osteoradionecrosis. By linking physics, radiobiology, and clinical manifestations, this chapter equips learners with a unified framework to understand radiation behavior, optimize diagnostic exposures, and anticipate radiotherapy-related complications.