Developed by the Working Group on the National Criminalisation of Ecocide, this Manual provides a comprehensive guide for European legislators and policymakers to incorporate ecocide as a criminal offense within national legal frameworks. Building on the European Union’s 2024 Environmental Crime Directive (ECD), which mandates Member States to criminalize severe environmental harm, the manual offers practical solutions to address challenges in implementing ecocide laws. Key topics include defining core elements such as unlawfulness, mens rea, thresholds of environmental harm, and extraterritorial jurisdiction. The manual also explores integrating ecocide as either a standalone crime or part of broader environmental offenses. Additionally, it highlights the need for clear definitions of terms such as “unlawful” and “substantive legal requirements” to ensure legal certainty and robust environmental protection. Drawing on international examples and expert insights, the manual aims to empower EU Member States to combat ecological destruction effectively and align with global efforts to recognize ecocide as a crime.