Radiation dose reconstruction refers to the process of estimating radiation doses that were received by individuals or populations in the past as a result of particular exposure situations of concern. The basic principle of radiation dose reconstruction is to characterize the radiation environment to which individuals have been exposed using available information. In cases where radiation exposures can not be fully characterized based on available data, default values based on reasonable scientific assumptions can be used as substitutes. The extent to which the default values are used depends on the purpose of the reconstruction(s) being undertaken. Radiation dose reconstruction refers to the process of estimating radiation doses that were received by individuals or populations in the past as a result of particular exposure situations of concern. The basic principle of radiation dose reconstruction is to characterize the radiation environment to which individuals have been exposed using available information. In cases where radiation exposures can not be fully characterized based on available data, default values based on reasonable scientific assumptions can be used as substitutes. The extent to which the default values are used depends on the purpose of the reconstruction(s) being undertaken. The methods and techniques used in dose reconstructions have been growing and evolving rapidly. It wasn’t until the late 1970s that dose reconstruction emerged as a scientific discipline and it has been used in practice in the United States for the last two decades. The scientific methods and practices used to complete dose reconstructions are often based on the standards published by international consensus organizations such as the International Commission on Radiological Protection. When conducted properly, dose reconstruction is a scientifically valid process for estimating radiation dose received by an individual or group of individuals. It is commonly used in occupational epidemiological studies to determine the amount of radiation workers may have received as part of their employment. For these types of studies, dose reconstruction is similar to the process of estimating how much radiation current workers receive, for example at a nuclear facility, except dose reconstructions evaluate past exposures. The terms historical and retrospective often are used to describe a dose reconstruction. Dose estimation is the term sometimes used to describe the process used to determine radiation exposures to current populations or individuals. Dose reconstruction methods have also commonly been applied in environmental settings to assess radionuclide releases into the environment from nuclear sites. One such environmentally focused study was published in 1983 by the U.S. Nuclear Regulatory Commission entitled Radiological Risk Assessment: A Textbook on Environmental Dose Analysis. This book was updated with major revisions in 2008 and it details the steps of radiological assessments, which uses similar methods and techniques as a dose reconstruction.