The paper considers whether – and under what conditions – private international litigation based on the duty of care can become a new tool for environmental regulation. Focusing on the French law of 2017 on the duty of care of parent companies and contractors, as well as on the European Commission proposal for a directive on corporate sustainability due diligence, the paper describes, in its first part, the new environmental due diligence obligations imposed on companies and the private enforcement mechanisms put in place to ensure their effectiveness. The second part of the paper brings to light the challenges that must be met, in terms of the procedural law of the States and their rules of private international law, if this private enforcement is to be effective.