AbstractUnderstanding the biology of alien invasive species (AIS) is a problem for pure, as well as applied biology. However, it is obviously not possible to conduct designed experiments to further our understanding. We can, however, study actual invasions; the next best thing. We used historical data from New Zealand to explore the correlates of successful establishment of species in a new environment. Surprisingly, biological differences between species explained little of the variation in establishment success. Instead, what matters is how often, and in what numbers, a species is introduced into the new environment.