During the last two decades, there was considerable debate on the actual number of insect species on Earth, and current estimates range from 5 to 30 million species. For gall-inducing insects, a cosmopolitan group of specialist herbivores, the last, 40-yr-old estimate of global richness indicated 13,000 species, mostly from temperate regions. To update these estimates, we used 10 studies that provided the average number of gall-inducing insect species per plant species (galled and nongalled) in different vegetation types around the world, and a recent calculation of 422,000 seed plant species on Earth. Considering that most of the gall-inducing insects are monophagous, these data were extrapolated to the global flora, assuming a positive relationship between gall-inducing insect richness and plant richness. Estimates of the global richness of gall-inducing insects ranged from 21,000 to 211,000 species, with an average of 132,930 species. The global distribution pattern described for gall-inducing insects pointed to a higher richness in warm temperate regions with sclerophyllous vegetation. However, it is likely that the tropical rain forests would harbor a higher absolute number of gall-inducing insects, due to their outstanding plant richness. Most of this fauna will be found on the poorly explored canopies of rain forests, especially in the Neotropical region. An increased sampling effort in tropical regions should fill the taxonomy gap represented by gall-inducing insects and possibly change the biogeographic patterns described so far.