A review of the national environmental legislation in each ASEAN member state illustrates that ASEAN displays both centrifugal and disparate tendencies in legal integration. ASEAN cooperation seeks harmonization and integration to facilitate steps toward region-wide environmental sustainability. However, individual states are driven by their historical and developmental legacies; priorities for ASEAN's sustainability are not always the same for national leaders. Most have not attained the recommended actions described in Agenda 21 to be considered sustainable societies, much less a model for sustainable development. Because of limitations in development state economies, problems with corruption and insufficient expertise, among other challenges, it may be unrealistic to assess legal integration in ASEAN solely from a classical model of international law. Conversely, if ASEAN member states chose to do so, the international environmental law agreements to which they are members provide them with many cooperative tools and legal frameworks for resolving environmental problems and attaining sustainable development. ASEAN could collaborate with its member states to avail themselves of such opportunities.

As sovereign states, international law regards each as equal, but as geographic countries, each has unique ecological challenges that differ substantially from state to state. Hence, national and sub-national environmental laws, and implementation of those laws, can be expected to vary greatly. Like all nations, the ASEAN member states may have enacted comparable bodies of environmental legislation through national law, and each may be a member of the various international treaties and multinational environmental agreements, but their capacity to implement treaty obligations also varies greatly. This reality is illustrated by a selective survey of some of the environmental legal issues in different ASEAN member states.

4.1 Southeast Asian legal systems and constitutions

With the establishment of colonies and protectorates in the region by Great Britain during the late eighteenth and nineteenth centuries, Brunei, Malaysia, Myanmar and Singapore came to inherit common law from the British legal system during their colonial rule. From their French colonial rule, the Indochinese countries (Vietnam, Lao PDR and Cambodia) acceded to a civil law system, though administered with socialist law since gaining their independence. Indonesia practices a civil law system based on a Roman-Dutch model influenced by customary (adat) law, while the legal system in the Philippines is a blend of civil law and common law from its Spanish and American rule, though common law is dominant.