Due Diligence in International Law identifies due diligence as the missing link between state responsibility and international liability. Acknowledged in all legal fields, it ensures international peaceful cooperation and prevents significant transboundary harm, yet it has thus far not been comprehensively discussed in literature. The present volume fills this void. Kulesza identifies due diligence as a principle of international law and traces its evolution throughout centuries. The no-harm principle, key to identifying responsibility for transboundary harm, focal to international environmental law and applicable to e.g. combating terrorism, follows states’ obligation of due diligence in preventing foreign harm. This obligation, present in various treaty-based and customary regimes is argued to be a principle of international public law applicable to all obligations of conduct.