The history of man's use of arid lands is a sad record of deterioration of the natural resource base and of low and declining living standards for the 300 million people who live in them. One prerequisite to meeting the challenge of reversing the deterioration and of raising living standards is a sound knowledge of the natural ecosystems. It is to this end that the IBP and other relevant studies, reported in this and a succeeding volume, are addressed. The subject matter of the two volumes is organised into five major topics; two are dealt with in the first volume. The first section of volume one describes the structure of arid ecosystems in terms of climate, soils, geomorphology, hydrology, flora and fauna. All continents except South America are covered. In the second section the processes which operate within, and control, the ecosystem are dealt with individually. This comprehensive account will be of importance to university teachers and professional ecologists throughout the world.