Genocide is the deliberate and systematic destruction, in whole or in part, of an ethnic, racial, religious, or national group.

The preamble to the CPPCG states that instances of genocide have taken place throughout history, but it was not until Raphael Lemkin coined the term and the prosecution of perpetrators of the Holocaust at the Nuremberg trials that the United Nations agreed to the CPPCG which defined the crime of genocide under international law.

There was a gap of more than forty years between the CPPCG coming into force and the first prosecution under the provision of the treaty. To date all international prosecutions of genocide, for the Rwandan Genocide, the Srebrenica Genocide, have been by ad hoc international tribunals. The International Criminal Court came into existence in 2002 and it has the authority to try people from the states that have signed the treaty, but to date it has not tried anyone.

Since the CPPCG came into effect in January 1951 about 80 member states of the United Nations have passed legislation that incorporates the provisions of the CPPCG into their municipal law, and some perpetrators of genocide have been found guilty under such municipal laws, such as Nikola Jorgic ,who was found guilty of genocide by a German court (Jorgic v. Germany).

Critics of the CPPCG point to the narrow definition of the groups that are protected under the treaty, particularly the lack of protection for political groups for what has been termed politicide (politicide is included as genocide under some municipal jurisdictions). One of the problems was that until there was a body of case law from prosecutions, the precise definition of what the treaty meant had not been tested in court, for example, what precisely does the term "in part" mean? As more perpetrators are tried under international tribunals and municipal court cases, a body of legal arguments and legal interpretations are helping to address these issues.

Another criticism of the CPPCG is that when its provisions have been invoked by the United Nations Security Council, they have only been invoked to punish those who have already committed genocide and been foolish enough to leave a paper trail. It was this criticism that led to the adoption of UN Security Council Resolution 1674 by the United Nations Security Council on 28 April 2006 commits the Council to action to protect civilians in armed conflict and to protect populations from genocide, war crimes, ethnic cleansing and crimes against humanity.

Genocide scholars such as Gregory Stanton have postulated that conditions and acts that often occur before, during, and after genocide— such as dehumanization of victim groups, strong organization of genocidal groups, and denial of genocide by its perpetrators— can be identified and actions taken to stop genocides before they happen. Critics of this approach such as Dirk Moses assert that this is unrealistic and that, for example, "Darfur will end when it suits the great powers that have a stake in the region".