While there is no requirement of ‘zero transboundary impact’ at international law, which would be unrealistic and incapable of being met by any State, developed or developing, there are both substantive and procedural obligations that penetrate within the State in the exercise of control over activities, which cause, or are likely to cause, significant transboundary harm, and which in the exercise of due diligence must be subject to internal environmental controls, including EIA, to ensure the protection of humans and of the environment. These derive both from general principles of international environmental law and more specifically from the host of bilateral, regional and global instruments for pollution control, one object of which is prevention of transboundary harm. In particular, international law requires that States prevent significant transboundary harm, which is widely recognised as imposing an obligation of the reasonable exercise of due diligence, the assessment of which is flexible and context-specific. One of the functions of treaty-based rules and standards is to provide concrete standards against which State compliance may be assessed, including obligations of result, and an institutional framework for cooperation. Marine and nuclear pollution standards and liability are amongst the most developed with multilateral regulatory regimes reducing reliance on general principles and the uncertainty (and cost and time) of international litigation. Air pollution – and protection of the atmosphere more generally – is less well regulated by treaty and in the absence of negotiated agreement is, for good or for ill, more apt to be subject to general principles.