INTRODUCTION Fallout from nuclear weapons tests has been by far the principal man-made source of radioactive environmental contamination. About 340 nuclear detonations in the atmosphere, by all nations testing, have been announced. Thetotal energy release has been about 511 million tons (MT) equivalent of TNT with the U.S.S.R. tests accounting for about 70 percent of the total.! Included in this total is about 193 million tons of energy released by fission—the process that creates the radioactive fission products present in fallout.!. Two hundred million tons of TNT energy equivalent would produce about 12 tons, by weight, of fission product debris. The discussion that follows in section I attempts to summarize an enormous amount of data and to present some evaluation of the estimated radiation exposures to persons from radioactive fallout. Section II deals with other health aspects of nuclear weapons testing. The information presented herein is intended to provide some answers to three basic questions concerning the testing of nuclear weapons: 1. What are the problems and possible risks associated with nuclear weapons testing? 2. What are the data concerning effects from past tests? 3. What do these data mean—howserious are the possible risks? With these three questions in mind, the information for each health aspect—such as whole body exposures—is presented under three subheadings, i.e., Background Information, The Data, and Evaluations.