The orang-utan is a superb representative of a major sector of the structure of biological diversity in Sumatra and Borneo. Conservation of the living conditions of the orang-utan implies maintaining the integrity of the entire natural ecosystem of indigenous plant and animal species known as the West Malesian rainforest, i.e. the natural tropical evergreen forests of distinctive floral composition which stretch from the isthmus of Kra, in Thailand, across peninsular West Malaysia, south and eastwards, including Sumatra, Borneo and Java. The main question behind this study is: What is the current status of the orang-utan? Or in other words: • what is the current geographical distribution range? • what have been the trends in the size of its range and numbers? • to what extent is this range covered by (a) conservation areas, (b) timher concessions (i.e. modified habitat) and (c) plans for conversion (i.e. obliteration of the habitat)? • what is the current quality of habitat in this range and what is the prospect for conservation or restoration of such habitat? • what is a plausible average density of the ape in such habitat? • what are the prospects for protection of the ape? • what should and can be clone to give the ape a chance of survival? Several actions were undertaken to find answers to these questions, and the major results are: • In Sumatra the orang-utan has a much more extensive range than was hitherto