Southeast Asia comprises a heterogeneous grouping of countries in terms of economic systems, level and structure of economic development, development problems and prospects, as well as in the noneconomic dimensions. The region includes the market oriented ASEAN countries of Indonesia, Malaysia, the Philippines, Singapore and Thailand, the inward-looking socialist economies of Kampuchea, Laos, Vietnam and Burma, and the small, oil-rich, sultanate of Brunei. Table I highlights the diversity of Southeast Asia in terms of population size and land area. Excluding Brunei, which has the smallest population and the highest per capita income in the region, Southeast Asia is confronted by two extremes. At the one extreme is Indonesia, with the fifth largest population in the world, a land area of 1.9 million square kilometres and a per capita Gross National Product (GNP) of under US$200. At the other extreme is the city-state of Singapore with a population of 2.3 million people concentrated within a land area of 588 square kilometres, and enjoying a per capita GNP of over US$2,500. This paper is divided into two main sections. In the first section, the development performance and development problems of the Association of Southeast Asian Nations (ASEAN) countries and the socialist economies, particularly in the 1976-77 period, are discussed. In the second section, developments in ASEAN economic co-operation since the Bali Summit of February 1976 are examined.