The Moscow theater hostage crisis, also known as the 2002 Nord-Ost siege,was the seizure of a crowded Moscow theater on October 23, 2002 by about 40-50 armed Chechen terrorists who claimed allegiance to the separatist movement in Chechnya. They took 850 hostages and demanded the withdrawal of Russian forces from Chechnya and an end to the Second Chechen War. The siege was officially led by Movsar Barayev (aged 22).

After a two-and-a-half day siege, Russian Spetsnaz forces pumped an unknown chemical agent into the building's ventilation system and raided it. Officially, 39 of the terrorists were killed by Russian forces, along with at least 129 and possibly many more of the hostages (including nine foreigners). All but one of the hostages who died during the siege were killed by the toxic substance pumped into the theatre to subdue the militants.