Although there is wide agreement that there are jus cogens prohibitions against the commission of war crimes, crimes against humanity and genocide, there is significantly less clarity regarding obligations to prevent such atrocities. This paper explores the existing prevention framework and is intended to speak to those States that seek to take preventive steps, understand their legal obligations and, most importantly, to operationalize those obligations by taking concrete action. It outlines what is known regarding the duty to prevent each international crime under both treaty and customary international law, while elucidating the gaps that make these obligations so difficult to enforce. This paper argues that States’ existing obligations require them to examine their capacity to prevent, and that one form of examination, which should be adopted as a matter of best practice, is to undertake an Atrocity Impact Assessment, evaluating both a State’s current impact in a country or region, and the potential measures it could take to assist in averting mass atrocities.