The quest for nuclear energy began with the discovery of radioactivity at the end of the 19^th century. Numerous experiments with materials emitting invisible radiation led to a better understanding of the structure of the atom. Ionising radiation is a natural part of our environment. It originates from naturally occurring radioactive elements in the Earth’s crust, air, food and living organisms, as well as from cosmic radiation. Everyone is therefore exposed to a certain amount of ionising radiation from natural sources, known as natural background radiation. Additional exposure may result from various diagnostic and medical procedures involving ionising radiation. Natural background radiation varies from place to place and is a normal part of the radiation exposure experienced by humans.