States are required to implement grave breaches within their domestic criminal law. The obligation to enact legislation necessary to provide effective penal sanctions in relation to grave breaches lies at the heart of any meaningful prosecution of grave breaches of the Geneva Conventions. Knowing what is required of states and understanding the different models of implementation is essential. Yet, despite its importance, this specific obligation has led a somewhat shadowy existence, often neglected in state practice and academic research. It is against this background that the present contribution aims to bring into focus the scope and precise content of this somewhat ambiguously formulated obligation.