Crimes against humanity are widespread or systemic criminal acts which are committed by or on behalf of a de facto authority, usually by or on behalf of a state, that grossly violate human rights. Unlike war crimes, crimes against humanity do not have to take place within the context of wars, and they apply to widespread practices rather than acts which are committed by individuals. Although crimes against humanity apply to acts which are committed by or on behalf of authorities, they do not need to be part of an official policy, and they only need to be tolerated by authorities. The first prosecution for crimes against humanity took place during the Nuremberg trials. Initially considered for legal use, widely in international law, following the Holocaust, a global standard of human rights was articulated in the Universal Declaration of Human Rights (1948). Political groups or states that violate or incite violations of human rights norms, as they are listed in the Declaration, are expressions of the political pathologies which are associated with crimes against humanity.

Since the Nuremberg trials, crimes against humanity have been prosecuted by other international courts (such as the International Criminal Tribunal for the former Yugoslavia, the International Criminal Tribunal for Rwanda, the Special Court for Sierra Leone, the Extraordinary Chambers in the Courts of Cambodia, and the International Criminal Court) as well as by domestic courts. The law of crimes against humanity has primarily been developed as a result of the evolution of customary international law. Crimes against humanity are not codified in an international convention, so an international effort to establish such a treaty, led by the Crimes Against Humanity Initiative, is currently underway.

Unlike war crimes, crimes against humanity can be committed during peace and war. They are not isolated or sporadic events because they are part of a government policy (although the perpetrators do not need to identify themselves with this policy) or they are part of a widespread practice of atrocities which is tolerated or condoned by a government or a de facto authority. War of aggression, war crimes, murder, massacres, dehumanization, genocide, ethnic cleansing, deportations, unethical human experimentation, extrajudicial punishments including summary executions, the use of weapons of mass destruction, state terrorism or state sponsorship of terrorism, death squads, kidnappings and forced disappearances, the use of child soldiers, unjust imprisonment, enslavement, torture, rape, political repression, racial discrimination, religious persecution and other human rights abuses may reach the threshold of crimes against humanity if they are part of a widespread or systematic practice.