There is a paradigm that links increased propagule pressure to invasion success. At the same time, some authors have suggested that nonindigenous species must overcome a genetic paradox, in which introduced species succeed despite low genetic diversity associated with drift and extinction. Molecular genetics can be used to analyze the role of propagule pressure in the establishment and spread of these species. Contrary to the expectations of the founder effect, recent analyses show that many invasive populations may not be subject to genetic drift and low diversity. The European green crab (Carcinus maenas) in eastern North America highlights the shift from an apparent single-event invasion in the nineteenth century, with severe founder effects, to a recent release with elevated levels of genetic diversity, indicating high propagule pressure. Although it had previously been assumed that the green crab spread north from New England to the Canadian Maritimes, genetic analysis indicates that multiple lineages were released in the Maritimes in the 1980s, rapidly expanding the range of this species. Such high propagule pressure can deliver genetic variation on a level comparable to that of native populations. The combination of increased propagule pressure and elevated diversity from high-volume releases such as ballast-water discharge may be responsible for the recent rise in invasions in aquatic environments. Recurrent migration events not only increase the probability of establishment of new species, but they can also bring novel physiological adaptations and expand the range of long-established nonindigenous species.