The PhD thesis explores individual criminal and state responsibility for omissions in the context of international crimes. In the framework of individual criminal responsibility, the thesis places a particular focus on exploring omission as the conduct of a crime and as an element that may give rise to different modes of criminal liability. In the sphere of state responsibility, the thesis gives great importance to the analysis of the preconditions that need to be fulfilled for a failure to prevent international crime to amount to omission, which may lead to state responsibility.