790 p.The aforementioned PhD addresses the difficult task of establishing the criminal liability of each of the numerous individuals who take part in the perpetration of atrocity crimes, which are massive by nature. In order to achieve such a goal, the thesis is divided into three parts.The first part is dedicated to the criminological analysis of the reality of atrocity crimes, and it studies the various factors which have an effect on the aetiology of such type of crimes. These factors are analyzed at three levels (macro, meso and micro), and a specific chapter is dedicated to the aetiology of the crime of genocide. The second part is the longest one. It is aimed at studying the different modes of liability in International Criminal Law: direct perpetration, co-perpetration, indirect perpetration, indirect co-perpetration, participation, and the Responsibility of Superiors. With that purpose in mind, the second part is based on a comparative analysis of the case law of the ad hoc tribunals and of the International Criminal Court. The present PhD supports the theory of control over the crime, in particular, its third expression: indirect perpetration through control over the will. The adequacy in International Criminal Law of indirect perpetration and indirect co-perpetration through an Organized Power Apparatus has been outlined.The third part combines the results of the criminological analysis with the knowledge regarding the modes of liability, and it suggests a general frame of modes of liability which can be applied to every individual who takes part in the perpetration of atrocity crimes. Furthermore, it suggests that such modes of liability are taken into consideration at the sentencing phase