Nuclear weapon is an extremely spectacular and at the same time lethal means of fighting. Some consider it a single "true” weapon of mass destruction whose destructive power exceeds everything that has ever been invented. Long lasting nuclear armament race was stopped, while countries that have nuclear weapon undertook many initiatives to its reduction and non-proliferation. A particular role is played by two strongest nuclear countries - the United States and Russia, which competing for many years were also the initiators of numerous disarmament treaties, pacts and conventions. The article presents the idea of contemporary threats in the aspect of nuclear weapon acquisition, possessing and using. For better understanding, the current world nuclear potential and its influence on international security is discussed. Also the revaluations that took place in the perception of threat awareness and defence methods are described. Four categories when nuclear weapon may pose a threat for the modern world are defined. They involve nuclear terrorism, new nuclear countries and regional conflicts, existing nuclear weapon arsenals and the risk that the non-proliferation idea of nuclear weapon may devaluate. Additionally, the latest solution of this problem is presented, called universal compliance, supported by Canegie Endowment for International Peace.