1. The South and Middle American tropics contain the world's largest area of moist savanna. Despite an apparent uniformity in appearance, floristic groupings can be detected and four zones are provisionally outlined with a number of characteristic plants.2. Although currect knowledge of their biogeography and palaeoecology is scattered and incomplete, it is evident that there have been significant changes throughout the neotropics since the glacial maximum.3. The largest and most significant savanna formations are the Brazilian cerrados, which occupy nearly 23% of that country, an area equivalent to that of western Europe. The cerrados range from savanna woodlands (cerradao), through progressively less arboreal formations (cerrado), to nearly treeless grasslands (campo). A number of adjoining vegetation formations are closely related, including evergreen and deciduous forests, scrub woodlands and wetlands.4. Until the last decade, the biological diversity of the neotropical savannas was thought to be limited since, structurally, they appeared to form a relatively simple mosaic of repeated physiognomic units. New evidence reveals a rich diversity of tree, shrub and herbaceous plants in a continuously changing spatial pattern. Such diversity reflects the great area of the biome, together with local differences in topography and drainage, soil conditions and fire frequency.5. Research on the dynamic processes affecting savannas is of increasing importance and urgency given the potential value of the area that is still relatively undisturbed (about 50% of the original, but with less than 2% having federal reserve status) and the pace of current conversion to different forms of land use. © 1999 Blackwell Science Ltd.