The Llanos Orientales of Colombia are between 3° and 7° lat N, extending as a level plain between 600 and 200 m above sea level, east of the Andes Mountains. The hot tropical climate shows a seasonal pattern of rainfall, divided between one rainy season and one dry season. In the survey of a 45,000—square—mile study area within this llanos region, 10 savanna types were recognized and classified on a floristic basis. Three of these types were floristically distinct, but with the homologous factor of all having relict forests. The other seven savanna types intergraded along a gradient of increasing humidity, and for convenience were divided into dry, humid, and inundable savannas. A great number of species were shared by these last seven types, their distinction being based on the relative dominance of species in the different types. The various theories on the origins of tropical savanna are discussed in the light of this new information on vegetation and that found in a concurrent soil survey. No single factor is believed responsible for savanna formation, but deflection from forest development is considered the result of several adverse factors, with savanna formation accelerated once the process has started.