The development of nuclear power was motivated by a geopolitical conflict, concretely World War II, and the race towards nuclear weapons in that conflict. Until today, the sector is still dominated by geopolitical considerations, even though the commercial uses are much more talked about in public. This chapter provides an overview of the role of nuclear power in geopolitical conflicts since the 1940s, and it summarizes the literature on classifying the countries that have or aspire to have nuclear weapons, and/or use nuclear power for commercial energy production. To do so, the paper traces the origins of nuclear power as the “child of science & warfare,” summarizes the history of the “first” and the “second” atomic age, describes the 40 or so nuclear countries, and explains main motivations of new entrant countries (“newbies”). We conclude that the geopolitical aspects of nuclear power are dominant and should receive more weight in discussions dominated by commercial nuclear power.