This article is focused on nuclear transboundary issues between Portugal and Spain, two countries that share a long history of nuclear collaboration and conflict of late, where national borders played a crucial role. The issues at stake cover the full spectrum of the nuclear cycle: uranium mining, power production and waste disposal. The first stage, under two fascist dictatorships, was characterised by collaboration within a common techno-political imaginary, where nuclear energy was understood as a driver of modernity, but with the absence of the public in decision-making processes. The second stage was marked by the advent of democracy in both countries and the reconfiguration of nuclear policies: while Portugal abandoned the nuclear endeavour, Spain implemented a nuclear moratorium but kept ten reactors operative. The third phase, which started in 1986 and goes until the present time, was marked by two crucial events: joining the European Communities (EC) and the Chernobyl accident. The first event allowed Brussels to become a referee on Spanish/Portuguese nuclear disputes. The second one implied that Portugal expanded its institutional vigilance on Spanish nuclear activities and led to the emergence of transboundary social movements against nuclear power.