Cities are often thought of as concrete jungles, not capable of supporting biodiversity. In this study, we examined the birds in 54 cities and plants in 110 cities worldwide. We found that cities support surprisingly high numbers of plant and bird species. However, cities have also lost significant biodiversity with urbanization. The most common species worldwide are pigeons and annual meadow grass. As urbanization continues to expand, conservation of blocks of intact vegetation within cities could enhance biodiversity. Despite declines in species, cities still retain endemic native species, providing opportunities for regional and global biodiversity conservation, restoration, and education.