[en] The production of ionising radiation from the naturally occurring radioactive decay chains is reviewed and mathematical expressions relating to secular equilibrium and the calculation of the activity of daughter products are introduced. The absorption of α, β and γ radiation is discussed from the point of view of the physical processes which occur, e.g. the photoelectric, Compton and pair production processes for γ-rays. Linear energy transfer (LET) and range-energy relationships are discussed for α and β particles. Units of measurement for ionising radiation, relative biological effectiveness, dose equivalence and quality factors for each type of radiation are reviewed. The behaviour and properties of radon, thoron and their daughter products are described, and the definition of the Working Level introduced