This chapter highlights the interaction of different radiations with matter. For particle interactions, ionization and excitation of absorber atoms, LET, Braggs ionization, Cerencov radiation and range have been discussed. For photon interaction with matter, photoelectric effect, Compton scattering, and pair production are discussed at length. Bremsstrahlung and annihilation processes are alluded to. In addition, attenuation, Raleigh scattering, and scattering of photons in absorbers are important and a brief discussion is provided. Half-value layer (HVL) and tenth-value layer (TVL) for different radionuclides in lead are provided in a table 6.2. Attenuation coefficients and mass attenuation coefficients are defined.