Plants in over 100 families bear extrafloral nectaries (EFNs), which secrete a carbohydrate-rich food that attracts ants and other arthropods. By fostering ecologically important protective mutualisms, EFNs play a significant role in structuring both plant and animal communities. As an introduction to a Highlight collection of papers published in the June issue, Marazzi et al. provide an overview of recent research on EFN diversity, ecology and evolution, and conclude that our understanding of the roles EFNs play in plant biology is being revolutionized with the use of new tools from developmental biology and genomics, new modes of analysis allowing hypothesis-testing in large-scale phylogenetic frameworks, and new levels of inquiry extending to community-scale interaction networks. The authors highlight major gaps in our current knowledge, and outline directions for future research.