The Moscow theater hostage crisis (also known as the 2002 Nord-Ost siege) was the seizure of the crowded Dubrovka Theater by Chechen terrorists on October 23, 2002, which involved 850 hostages and ended with Russian security services killing or causing the death of at least 170 people. The attackers, led by Movsar Barayev, claimed allegiance to the Islamist separatist movement in Chechnya. They demanded the withdrawal of Russian forces from Chechnya and an end to the Second Chechen War.

Due to the layout of the theater, special forces would have had to fight through 30 meters (100 ft) of corridor and advance up a well-defended staircase before they could reach the hall in which the hostages were held. The attackers had numerous explosives, with the most powerful in the center of the auditorium. Spetsnaz operators from Federal Security Service (FSB) Alpha and Vympel, supported by a Russian Ministry of Internal Affairs (MVD) SOBR unit, pumped a chemical agent into the building's ventilation system and began the rescue operation.

All 40 of the insurgents were shot dead when unconscious, and up to 130 hostages died during the siege, including 9 foreigners, due to the toxic substance pumped into the theater. The identity of the gas was not disclosed at the time, although it was believed by some to have been a fentanyl derivative, such as carfentanil.

A study published in 2012 concluded that it had been a mixture of carfentanil and remifentanyl. The same study pointed out that in a 2011 case at the European Court of Human Rights, the Russian government stated that the aerosol used was a mixture of a fentanyl derivative and a chemical compound with a narcotic action.

The number of estimated casualties varies widely because many hostages remained unaccounted for and were not included in the official list. Some estimates have put the civilian death toll at more than 200 with 204 names on one list, or even 300, including people who died during the year after the siege from complications from the poison gas. Some former hostages and relatives of the victims claim that the death toll from the chemical agent is being kept secret. According to official numbers, 40 terrorists and about 130 hostages died during the raid or in the following days.

Among the fatalities, 17 were Nord-Ost cast members, including two child actors. Of the foreign nationals, three were from Ukraine, and the others were citizens of Austria, Armenia, Belarus, Kazakhstan, the Netherlands, and the United States.