This work is concerned to the applicability of this principle to the crime of genocide: The case of Genocide against the Tutsi. This principle requires that no crime or punishment can exist without a legal ground. This work is trying mention how this principle has been taken as defense under international law as well as in domestic law. As this book specifically talks about about the case of Genocide in Rwanda, in 1994, and of course there were no law that punishes the crime of Genocide, by finding an answer that matter at hand, new law was adopted that punishes the perpetrators of Genocide. This new law adopted was greatly criticized to be retroactive which is prohibited in the principle of legality. But this book shows well that the crime of Genocide is an international crime, and must be punished regardless whether there is a law or not because "killing" in itself is against a human being.