In response to the Copenhagen Accord and subsequent request by the 16th and 17th Conferences of the Parties (COP16 and COP17) to the United Nations Framework Convention for Climate Change for additional submissions, more than 50 developing countries have submitted information on their Nationally Appropriate Mitigation Actions (NAMAs). Many other developing countries are in the process of preparing their NAMAs. While there is a general understanding that NAMAs aim to achieve a deviation from business as usual (BAU) emissions in 2020 in developing countries, the definition of NAMAs has remained ambiguous, leaving ample room for different interpretations. This has led to diversity in submitted NAMAs in terms of type and scope, ranging from project-based actions to sector-based policy and measures, and to economy-wide targets. Despite ambiguity of the concept and diversity in the scope of submitted NAMAs, the core feature of NAMAs is that they are formulated “in the context of sustainable development”, and provide significant opportunities for low-carbon development in developing countries. Against this backdrop, this policy brief assesses the NAMA formulation process in selected Southeast Asian countries (Cambodia, Indonesia, Lao PDR, Thailand, Viet Nam), and analyses how much progress they have made in formulating NAMAs.