Protected areas are vital for life on earth. They safeguard biological and cultural diversity, help to improve the livelihoods of local communities, provide the homelands for many indigenous peoples and bring countless benefits to society in general. As the world becomes more crowded, and as the pressures on natural resources increase, so there is a growing recognition of the importance of such places to the future of humankind. But why should particular attention be given to transboundary protected areas - that is, to adjoining protected areas that involve a degree of co-operation across one or more boundaries between (or within) countries?It is now generally understood that conservation planning cannot just be site- specific, but has to be at broader scales, both at national and regional levels. Plants and animals do not recognize national boundaries; nor do many of the forces that threaten them. Clearly, strategies to conserve biodiversity in the 21st century must emphasize transboundary co-operation in relation to shared ecosystems and other conservation concerns.The rationale for this was graphically expressed by Dr Z. Pallo Jordan (then South African Minister of Environmental Affairs and Tourism) in his opening address to the 1997 Cape Town meeting on Transboundary Protected Areas: "The rivers of Southern Africa are shared by more than one country. Our mountain ranges do not end abruptly because some 19th century politician drew a line on a map. The winds, the oceans, the rain and atmospheric currents do not recognize political frontiers. The earth's environment is the common property of all humanity and creation, and what takes place in one country affects not only its neighbours, but many others well beyond its borders"Many countries have responded to this challenge. As these guidelines report, the numbers of transboundary protected areas have grown rapidly in recent times. In 1988, there were only some 59 groups of adjoining protected areas, separated by national boundaries. By 2001 this had grown to 169, involving 666 individual protected areas. In many cases there were co-operative arrangements in place: true transboundary protectedareas. While this is a welcome trend, there is a need to consolidate this experience.Quite apart from the benefits for biodiversity conservation, transboundary protected areas can also play an important role in fostering better co-operation and understanding between countries. Indeed they may help catalyze the peaceful resolution of disputes. In many parts of the world, transboundary protected areas have been important in building bridges between nations and peoples. But, here too, until recently at least, this experience had not been analysed systematically, nor had the lessons been drawn from it.