The article emphasizes that genocide was first criminalized immediately after the Second World War, precisely in 1948 when the UN General Assembly unanimously adopted the Convention on the Prevention of Genocide. In terms of severity, it is undoubtedly the most serious crime for both peacetime and wartime, and it is also the most difficult to prove. First of all, it is argued that in order to establish the very fact of genocide, it is necessary to have the intention to destroy, in whole or in part, any national, ethnic, racial or religious group, as well as for this destruction to be carried out by killing, inflicting grievous injuries, creating intolerable conditions life, prevention of childbirth or forced transfer of children to another group. The main focus of the article is on the fact that crimes against humanity most often occur during war. In order for the underlying crimes—such as murder, torture, extermination, sexual violence, and other acts—to qualify as crimes against humanity, they must be part of a widespread or systematic attack on civilians. Unfortunately, trials for genocide and crimes against humanity take forever. An example is the events in Srebrenica, proceedings in several genocide cases are still ongoing. Genocide, as well as crimes against humanity, are most clearly defined in legal terms in the Rome Statute of 1998, which became the foundational document of the International Criminal Court. Both crimes were also included in the criminal codes of many countries, the Criminal Code of Ukraine (Articles 442, 437) was not an exception. However, until now, there is no responsibility for crimes against humanity in national legislation. Therefore, currently the criminal actions of the combatants can only be qualified as war crimes. Many states have integrated the crime of genocide into national legislation, but not all have used the UN definition. Many states have also included crimes against humanity in their criminal codes. It is quite clear that both genocide and crimes against humanity are systematic crimes, to prove which it is not enough to confirm with evidence the commission of individual acts. Both crimes are committed within a system, often wide-ranging, and both contain elements that require proof of relevant intent or awareness on the part of the perpetrators, for example, in the case of genocide, by proving that the perpetrators knew of an express or implied plan.