To develop accurate estimates of the health risks resulting from atomic-bomb radiation exposure it is essential to have reliable estimates of the radiation doses received by individual atomic-bomb survivors. Because of uncertainties regarding the amount of radiation released by the bombs and how much of that radiation reached a specific location as well as uncertainties concerning the location and degree of shielding for individual survivors, dose assessment (dosimetry) is a continuing process that becomes more refined as a better understanding of the nature of the radiation produced by the bombs develops and as the quality of information available for individual survivors improves. The basic estimates of radiation dose and the general methods for applying these estimates to individual survivors are developed through an international collaboration that involves researchers in Japan, the United States, and Europe with approval provided by a group of experts from Japan and the United States. While RERF researchers have a role in the development of the basic dosimetry system, their main responsibility concerns implementing this system at RERF, using it to compute dose estimates for individual survivors, developing a better understanding of the magnitude and nature of uncertainties in these estimates, and collaborating in ongoing efforts to refine and improve dose estimates. The extensive, detailed information on location and shielding that were gathered by RERF staff over the past half century are crucial to dose estimation. The current dosimetry system, referred to as DS02, was introduced in 2002.