Eastern Brazil is a vast tropical area known to be rich in cactus diversity and endemism. Early explorers documented little of this due to the difficulty cacti present for preservation and description of living and museum specimens. Most taxa now known were discovered in the past 100 years, the majority being named and classified by horticulturists, whose activities greatly distorted their number and geographical range. Extensive literature, field and herbarium studies enabled better determination of generic and specific limits, correct nomenclature and details of distribution and ecology. Keys have been writte~ to facilitate identification of the 30 genera, 134 species and 43 heterotypic subspecies here recognised. In Eastern Brazil cactus biodiversity is concentrated in Bahia and Minas Gerais - each having > 90 taxa and 30 or more state endemics. The most likely explanation for this is greater complexity and discontinuity of habitats, leading to isolation of taxa and increased speciation. Cactus phytogeography lends support to recent hypotheses on historical plant migration routes in South America, while analysis of distribution patterns indicates> 94% of taxa as ± restricted to one of 3 major vegetation categories: (1) Mata atlantica, (2) Caatingas & Northern campos rupestres and (3) Cer~ados& South-eastern campos rupestres. Significantly, the campos rupestres are rich in endemic cacti, but divisable into two areas on the basis of cactus diversity. Of taxa included in category (2), > 70% belong to one of three major geographical-ecological areas, ranged west to east. Conservation status has been determined usmg information gathered for the above inventory and applying IUCN 1994 'Red List Categories' combined with Farjon & Page criteria (,phylogenetic distinction', 'ecological importance', 'genetic diversity'). As guidance to conservationists, prioritised shortlists of threatened taxa and Area Hotspots are provided. The South-eastern campos rupestres are of the greatest concern in terms of threatened taxa.