A D value is the quantity of radioactive material which is considered a dangerous source. A dangerous source is one that, if uncontrolled, could result in death or a permanent injury which decreases that person’s quality of life. Various IAEA documents concerning emergency preparedness and safety and security of radioactive sources list D values for a limited set of radionuclides. This document describes, in detail, the basis for the D-values given in various Agency documents and provides D-values for over 400 radionuclides that may be relevant in the event of a nuclear or radiological emergency.