Transboundary haze pollution is the Association of Southeast Asian Nation's (ASEAN) greatest environmental challenge to date. (1) Haze pollution from land and forest fires, largely in Indonesia, has repeatedly caused havoc in Malaysia and Singapore. These fires have far-reaching impacts, from causing significant damage to biodiversity, health and local economies to contributing to global climate change. (2) The worst forest fires were during 2013 and 1997-98, yet occur annually during the dry season with varied intensity. Humans are responsible for most of the fires, by clearing forest and lands for plantations and agricultural activity, especially in peatland areas. (3) Given the ongoing threat of transboundary haze pollution, its management and prevention has been listed as one of ten priority areas of regional importance for environmental cooperation for 2009 to 2015. (4) Finding a solution to transboundary pollution requires international and regional environmental law mechanisms, in addition to domestic legal systems that are more effective. (5) As international law provides frameworks and mechanisms for an appropriate response to transboundary haze, this article addresses two frameworks: customary international law and treaties. (6) The former involves the state responsibility principle, which comprises the duty to prevent transboundary pollution and to compensate accordingly. Implementation of the state responsibility principle is particularly problematic in Southeast Asia, as state sovereignty is strongly embedded in the Charter. (7) A further complication is the so-called ASEAN Way, a term that describes the preference of countries for non-interference in each other's domestic affairs, the use of consensus decision-making and cooperative programmes, and national implementation rather than reliance on a strong region-wide agency or bureaucracy. (8) The second framework for addressing haze pollution is Multilateral Environment Agreements or MEAs, namely treaties on atmosphere and biodiversity. These include the United Nations Framework Convention on Climate Change (UNFCCC), (9) and its Kyoto Protocol, (10) the Convention on Biodiversity (CBD), (11) and the Ramsar Convention. (12) MEAs are international frameworks, which can indirectly support the efforts in combating forest fires at the national level. However, there are constraints on the domestic implementation of MEAs in Southeast Asia, namely, overlapping domestic legal frameworks, lack of financial and human resources, and capacity building. Based on the two international legal frameworks mentioned above, this article will firstly examine the extent to which international law has influenced approaches in addressing transboundary pollution in the region. Secondly, it will assess whether international law has successfully contributed to regional environmental solutions. This article argues that further implementation of international law at the regional level is a necessary step forward. International Legal Regime Addressing Transboundary Air Pollution Transboundary air pollution has long been a concern of international law. (13) It has remained high on the environmental agenda in many regions including Europe, North America and Southeast Asia. (14) As transboundary air pollution is now a global threat that is contributing to issues such as ozone depletion and climate change, addressing it requires international cooperation. International law has responded to this threat by developing customary international law and MEAs on atmosphere and biodiversity as respective frameworks for its management. For customary environmental law, the principle of state responsibility for transboundary environmental harm, or the no harm principle, is particularly significant. Treaties also play an important role, especially those relating to atmosphere and biodiversity. Although these MEAs do not directly address land and forest fires, they can help to provide a framework for countries to protect their natural resources, prevent the transboundary spread of pollution and avoid international conflict. …