The stability of the Southeast Asian region has been affected by a number of unresolved territorial and maritime disputes because most parts of the region were not demarcated after the colonial period. Overlapping claims have led to disputes among ASEAN members for the rightful ownership of those areas. ASEAN, however, does not have the power to resolve disputes among its members or between its members and outsiders. The International Court of Justice (or the ICJ) has played an important role in ending long-standing disputes in the region; the ICJ’s judgements have normally been accepted. Thailand, which is an ASEAN member, has faced both territorial disputes due to the European colonists’ treaties over boundary agreements with its neighbouring states as well as maritime disputes due to the announcement of the 1983 United Nations Convention on the Law of the Sea (UNCLOS), particularly in the Gulf of Thailand. It seems to be easier to reach an agreement on the overlapping claims in the Gulf of Thailand than to settle disputes over land. Sharing the benefits of fisheries, oil and gas have contributed to Thailand and some claimants reaching an agreement by peaceful means. Territorial disputes are more complicated and sensitive than the maritime ones because they include strategic, ethnic and economic values, thus they are more likely to result in the use of military force which greatly harms the stability and peace of the region. Thailand has had many territorial disputes with its neighbours such as Myanmar, Laos, Malaysia and, particularly, with Cambodia. The rivalry between Thailand and Cambodia has existed since ancient times. One of the most important territorial disputes between the two nations is the territorial dispute over the sovereignty of the Preah Vihear temple which was awarded to Cambodia in 1962 by the ICJ. However, the 4.6 sq km area surrounding the temple has not yet been demarcated by the ICJ, thus the territorial dispute over the temple’s surrounding area occurred in 2008, and it was seen to be one of the bloodiest disputes in the region. The dispute has not yet ended. The ICJ has finally become involved and Thailand and Cambodia still need to provide evidence to the Court in 2013 to claim their rights over the disputed area. This dispute has caused ASEAN to become more concerned about its role and power to maintain peace in the region. ASEAN in the future must have more power to control such a situation so that it does not escalate into the use of military force among its members. This can prevent outsiders’ involvement and also improve trust among member states as well.