This encyclopedia covers the entirety of the Celtic world, both through time and across geography. Although emphasizing the areas where Celtic languages and traditions survive into the present, the work does not slight the reaches of the Celtic empire, which was the largest language and cultural group on earth prior to the rise of Rome. In some 1,500 articles, many representing original research by the finest Celtic scholars, the work covers the Celts from prehistory to the present, giving comprehensive treatment to all topics from myth to music, religion to rulers, literature to language, government to games, and all topics in between.

Entry into the rich world of Celtic scholarship has been difficult: The available information must be teased out of dense scholarly tomes or lightweight books emphasizing such popular elements of Celtic tradition as wee people, Druids, and blue-painted pagans. While this work treats those topics within its broad scope, the information is presented as scholarship, without the pop patina.