This report sets out to examine cross-border environmental co-operation and joint action on the environment facilitated by the Good Friday/Belfast Agreement, and what role if any the Good Friday/Belfast Agreement will have in maintaining this co-operation after Brexit. The institutions of Good Friday/Belfast Agreement co-operation, and the extent of ongoing environmental co-operation are sketched out. The different legal aspects of the Good Friday/Belfast Agreement as an instrument of international law and human rights law respectively are explored. Some of the impacts of Brexit on cross-border co-operation (in both a Withdrawal Agreement and “no-deal” scenarios) are briefly outlined. Finally, recommendations for preserving or enhancing cross-border environmental co-operation are put forward.