Clinical class 1 integrons are a major contributor to the evolution and dissemination of antibiotic resistance. The conserved motifs of these integrons suggest that a single, recent ancestor gave rise to all current variants. They have had a spectacular increase in distribution and abundance over the last 100 years, exhibiting many similarities to invasive species that prosper under human impacts. They have spread into over 70 bacterial species of medical importance, are commonly resident in the gut of humans and domesticated animals, and have invaded every continent, including Antarctica. They have done so via linkage with transposons, metal, disinfectant and antibiotic resistance genes. As a consequence of their invasive nature they have now become significant pollutants of natural environments.