Since the early 1990s, information on radiation-exposed populations other than those exposed from the Chernobyl accident in 1986 has become increasingly available for international scientific research. It is essential to understand how the cohorts of exposed populations have been defined and what mechanisms can be used to study their health outcomes. Different international scientific research collaborations currently investigate four population groups chronically exposed to ionizing radiation during the late 1940s and early 1950s in the Russian Federation and in Kazakhstan. In this framework, collaborations have been established to develop cause-of-death registers in each of these four areas for future mortality follow-up purposes with the aim of studying the health effects of ionizing radiation. The emphasis of this effort is on assessing the information sources available, the mechanisms of data collection and coding, and the data quality and completeness of the information collected. One of the major challenges is the harmonization of all these aspects between the four different centers to the extent possible, taking into account that much of the actual data has been collected over many decades.