"Generally speaking, genocide does not necessarily mean the immediate destruction of a nation, except when accomplished by mass killings of all members of a nation. It is intended rather to signify a coordinated plan of different actions aiming at the destruction of essential foundations of the life of national groups, with the aim of annihilating the groups themselves. The objectives of such a plan would be the disintegration of the political and social institutions, of culture, language, national feelings, religion, and the economic existence of national groups, and the destruction of the personal security, liberty, health, dignity, and even the lives of the individuals belonging to such groups."

This study was an elaboration of ideas he first proposed in 1933 in his address to the Fifth International Conference for the Unification of Penal Law (1933), which argued that attacks on racial, religious and ethnic groups should be considered international crimes. Important for the prosecution of the Nazis, it helped to establish the framework for all subsequent efforts to punish crimes against humanity.

When Lemkin proposed a treaty against genocide to the United Nations in 1945, he defined it as follows:

"The crime of genocide should be recognized therein as a conspiracy to exterminate national, religious or racial groups. The overt acts of such a conspiracy may consist of attacks against life, liberty or property of members of such groups merely because of their affiliation with such groups. The formulation of the crime may be as follows: "Whoever, while participating in a conspiracy to destroy a national, racial or religious group, undertakes an attack against life, liberty or property of members of such groups is guilty of the crime of genocide."