"Big" plant genera, those of 500 or more species, have not only occasioned interest among systematic botanists, but for geographical, ecological or horticultural reasons, have also become well-established popular concepts. Their size has rendered them difficult, if not impossible, to study in their entirety; there have been few full revisions since the nineteenth century. Despite their embodiment of significant taxonomic, biological and evolutionary questions, from the 1980s their importance has been more generally recognised, and recent technological and methodological developments have made it easier to come to grips with their study. This paper synthesizes growth impediments to our knowledge of these genera and possible approaches to studying them. Of the 57 genera currently thought to have 500 or more species, 22 of these were also considered in 1883 to be large (300 or more species). Others gained their prominence as a result of later exploration, some of it driven by enthusiasts. It is argued that comparisons of size, phenomena, and processes will only be possible by comprehensive study of the lineages comprising these genera and their immediate historical relatives; uncritical lists or relatively limited samples are not enough. Such studies will be of great scientific as well as cultural benefit.