Early Celtic-speakers left their traces in place-names from Ireland to Anatolia, from Scotland to the Apennines, and from Andalusia to the Black Sea. This original study reveals their full extent for the first time, exploiting a dataset of over 20,000 names recorded by Greek and Latin authors such as Polybius, Caesar and Tacitus and by early geographers such as Strabo, Pliny, Ptolemy and the Ravenna Cosmographer.