Alien species are the principal threat to the conservation of the Galápagos Islands, but little is known about the status of nonindigenous Galápagos insects and their effects on the biota. Currently, 463 alien insect species have probably been introduced to the Galápagos, an increase of 186 unintentional species introductions since an inventory in 1998. Alien insects now constitute 23% of the total insect fauna. Six species are known to be invasive and a threat to the biota: two species of fire ant and two wasps, a scale insect, and an ectoparasitic dipteran. The ecological impacts of the remaining species are unknown, making the prioritization of action for conservation management difficult. Thus, a newly developed and simple scoring system is presented to predict their potential invasiveness based on trophic functional role, distribution in Galápagos, and history of invasiveness elsewhere. An additional 52 species are predicted to be highly invasive. The endemic flora is most at risk because the largest proportion (42%) of the introduced species is herbivores. Plant populations are threatened principally by vectors of plant disease and by phloem and leaf feeders. Introduced predators and parasitoids (17%) may either be affecting, or have the potential to affect, the status of terrestrial invertebrate populations. At least 10% of the species are considered to be negligible threats to Galápagos ecosystems.