In the aftermath of a radiological emergency the public will see radiation and its potential hazards described in many different and sometimes confusing ways. This primer is intended to help journalists and community leaders understand these terms. Activity or radioactivity is measured by the number of atoms disintegrating per unit time. A becquerel is 1 disintegration per second. A curie is 37 billion disintegrations per second, which is the number of disintegrations per second in 1 gram of pure radium. A disintegrating atom can emit a beta particle, an alpha particle, a gamma ray, or some combination of all these, so becquerels or curies alone do not provide enough information to assess the risk to a person from a radioactive source. Disintegrating atoms emit different forms of radiation–—alpha particles, beta particles, gamma rays, or x-rays. As radiation moves through the body, it dislodges electrons from atoms, disrupting molecules. Each time this happens, the radiation loses some energy until it escapes from the body or disappears. The energy deposited indicates the number of molecules disrupted. The energy the radiation deposits in tissue is called the dose, or more correctly, the absorbed dose. The units of measure for absorbed dose are the gray (1 joule per kilogram of tissue) or the rad (1/100 of a gray). The cumulative dose is the total absorbed dose or energy deposited by the body or a region of the body from repeated or prolonged exposures. Alpha particles, beta particles, gamma rays, and x-rays affect tissue in different ways. Alpha particles disrupt more molecules in a shorter distance than gamma rays. A measure of the biologic risk of the energy deposited is the dose equivalent. The units of dose equivalent are sieverts or rem. Dose equivalent is calculated by multiplying the absorbed dose by a quality factor. Sometimes a large number of people have been exposed to a source of ionizing radiation. To assess the potential health effects, scientists often multiply the exposure per person by the number of persons and call this the collective dose. Collective dose is expressed as “person-rem” or “person-sieverts”