What regulates the size and distribution of plant populations? What determines whether a population of plants will increase in size or decline? What allows some species to become aggressive invaders of exotic habitats, and what prevents this from occurring? When do individuals from different species exclude each other from a community by competition, and when do their interactions facilitate coexistence? These are just a few of the questions that arise when one contemplates the ebb and flow of plants across our landscapes, the magnificent diversity one finds in some habitats, or the tendency of one species to exclude all others in habitats that would appear to be very similar. The latest issue of Annals of Botany is a Special Issue that brings together 15 articles presenting recent research bearing on these and other questions that have become, or continue to be, hot topics in the study of plant populations.