This chapter provides a thorough foundation in basic radiation science, which is essential for understanding the more advanced concepts discussed in subsequent chapters. Divided into seven sections, it begins with an atomic structure, the evolution of atomic theory and introducing fundamental concepts essential for understanding the nature of matter. The chapter discusses fundamentals of atomic structure, electromagnetism, radiation physics, and radioactivity, serving as the scientific foundation for radiological technology and imaging. It begins by examining why radiation is classified as an electromagnetic wave, transitioning into the historical evolution of atomic theory from ancient philosophical concepts to modern quantum models. Key contributors such as Dalton, Thomson, Rutherford, and Bohr are discussed, along with modern principles including quantum numbers, Pauli’s exclusion principle, and Hund’s rule. Essential atomic terms isotopes, isobars, isomers, and binding energy are defined for better understanding of nuclear behaviour. Section B introduces the fundamental principles of electricity and magnetism, essential for understanding X-ray generation. It includes Coulomb’s law, Lorentz force, and the concepts of voltage, current (mAs), and electromagnetic fields laying the groundwork for comprehending electromagnetic radiation. The chapter continues with detailed coverage of radiation quantities and units, including exposure, absorbed dose, kerma, effective dose, and equivalent dose, as well as their relevance in medical imaging. The chapter further discusses radiation-matter interactions including photoelectric effect, Compton scattering, and pair production. Subsequent sections discuss X-ray production mechanisms, soft and hard X-ray properties, and clinical applications. The chapter also addresses radionuclide production (reactor, cyclotron, and generator-based methods) and the science of radioactivity, including decay modes, half-life, and associated safety concerns.