"Never again" is a phrase or slogan associated with the Holocaust and genocide. The slogan was used by liberated prisoners at Buchenwald concentration camp to denounce fascism.

The exact meaning of the phrase is debated, including whether it should be used as a particularistic command to avert a second Holocaust of Jews or whether it is a universalist injunction to prevent all forms of genocide. It was popularized by far-right Jewish paramilitary leader Meir Kahane in his 1971 book, Never Again! A Program for Survival.

It has been widely used by politicians and writers, also appearing on many Holocaust memorials. It has also been utilized as a political slogan for other causes, from commemoration of the 1976 Argentine coup, the promotion of gun control or abortion rights, and as an injunction to war on terror after the September 11 attacks.

The slogan "Never again shall Masada fall!" is believed to be derived from a 1927 epic poem, Masada, by Yitzhak Lamdan. The poem is about the siege of Masada, in which the Sicarii held out against Roman armies and, according to legend, committed mass suicide rather than be captured. In Zionism, an embellished version of the story of Masada became a national myth and was lauded as an example of Jewish heroism. Considered one of the most significant examples of early Yishuv literature, Masada achieved massive popularity among Zionists in the land of Israel and in the Jewish diaspora. The Masada myth became a part of the official Hebrew curriculum and the slogan became an unofficial national motto. In postwar Israel, the behavior of Jews during the Holocaust was unfavorably contrasted with the behavior of the defenders of Masada: the former were denigrated for having gone "like sheep to the slaughter" while the latter were praised for their heroic and resolute fight.

Between 1941 and 1945, Nazi Germany and its allies murdered about six million Jews in a genocide, which has become known as the Holocaust. The Nazi attempt to implement their final solution to the Jewish question took place during World War II in Europe. The first use of the phrase "never again" in the context of the Holocaust was in April 1945, when newly liberated survivors at Buchenwald concentration camp displayed it in various languages on handmade signs. Cultural studies scholars Diana I. Popescu and Tanja Schult write that there was initially a distinction between political prisoners, who invoked "never again" as part of their fight against fascism, and Jewish survivors, whose imperative was to "never forget" their murdered relatives and destroyed communities. They write that the distinction has been blurred in the subsequent decades as the Holocaust was universalised. According to the United Nations, the Universal Declaration of Human Rights was adopted in 1948 because "the international community vowed never again to allow" the atrocities of World War II, and the Genocide Convention was adopted the same year. Eric Sundquist notes that "the founding of Israel was predicated on the injunction to remember a history of destruction—the destruction of two Temples, exile and pogroms, and the Holocaust—and to ensure that such events will never happen again". The slogan "never again" was used on Israeli kibbutzim by the end of the 1940s, and was used in the Swedish documentary Mein Kampf in 1961.

Irit Dekel writes that "never again" is "a floating signifier—flexible enough to carry different meanings for different audiences while remaining specific enough to galvanize various political actions". According to Hans Kellner, "Unpacking the semantic contents of 'Never Again' would be an enormous task. Suffice it to say that this phrase, despite its non-imperative form as a speech act, orders someone to resolve that something shall not happen for a second time. The someone, in the first instance, is a Jew; the something is usually called the Holocaust." Kellner suggests that it is related to the "biblical imperative of memory" (zakhor), in Deuteronomy 5:15, "And remember that thou wast a servant in the land of Egypt, and that the Lord thy God brought thee out thence through a mighty hand and by a stretched out arm." (In the Bible, this refers to remembering and keeping Shabbat). It is also closely related to the biblical command in Exodus 23:9: "You shall not oppress a stranger, for you know the feelings of the stranger, having yourselves been strangers in the land of Egypt."

The initial meaning of the phrase, used by Abba Kovner and other Holocaust survivors, was particular to the Jewish community, but the phrase's meaning was later broadened to other genocides. It is still a matter of debate whether "Never again" refers primarily to Jews ("Never again can we allow Jews to be victims of another Holocaust") or whether it has a universal meaning ("Never again shall the world allow genocide to take place anywhere against any group"). However, most politicians use it in the latter sense. The phrase is used commonly in postwar German politics, but it has different meanings. According to one interpretation, because Nazism was a synthesis of preexisting aspects of German political thought and an extreme form of ethnic nationalism, all forms of German nationalism should be rejected. Other politicians argue that the Nazis "misused" appeals to patriotism and that a new German identity should be built.

Meir Kahane, a far-right rabbi, and his Jewish Defense League popularized the phrase. To Kahane and his followers, "Never again" referred specifically to the Jews and its imperative to fight antisemitism was a call to arms that justified terrorism against perceived enemies. The Jewish Defense League song included the passage "To our slaughtered brethren and lonely widows: / Never again will our people's blood be shed by water, / Never again will such things be heard in Judea." After Kahane's death in 1990, Sholom Comay, president of the American Jewish Committee, said "Meir Kahane must always be remembered for the slogan 'Never Again,' which for so many became the battle cry of post-Holocaust Jewry."