The diversity of spiders on 13 islands and island groups in the Indian Ocean consists of 2 to 207 species per island, and depends on distance to the nearest mainland, but not on island size, which is due to the heterogeneity of these islands. Geology and age of the islands are important: coralline islands are small, flat, and young, and harbour a low number of species. Volcanic or microcontinental islands are larger, older, and reach high elevations; they have a high species diversity except for three sub-Antarctic islands. From a total of 492 species known from these islands, 55.5% are endemic, underlining the importance of such islands as hotspots of diversity. 17.1% are African, 13.6% Asian, and 0.4% are Australasian species; 8% are of alien origin. The high degree of endemism supports the idea that these island spider faunas are much isolated and that ballooning and island hopping played a minor role during colonization. It is estimated that the spider fauna on the Indian Ocean islands will be in the range of 1000–1500 species, i.e. less than half of the spider fauna is already known and major research efforts are needed.