A critical evaluation of more than 2,200 publications, some dating back to the late 1800s, established the presence, and traced the spatio-temporal spread, of 558 alien metazoan species in the Mediterranean Sea. The majority of aliens in the eastern Mediterranean entered through the Suez Canal, whereas mariculture and shipping are powerful means of introduction in the northwestern Mediterranean and in the Adriatic Sea. Most aliens are thermophilic species. The possible causes for the epic scale of invasion in the Mediterranean Sea are discussed.