This category brings together articles about closed administrative-territorial formations of Nizhny Novgorod Oblast — cities with restricted entry that have existed or still exist under a special secrecy regime. The foremost of them is city of Sarov, also known under its secret postal designations, including Arzamas-75. It is home to the Russian Federal Nuclear Center — All-Russian Research Institute of Experimental Physics) — one of the country's key nuclear research centers, which has defined the city's special status for decades to come.