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.

In 2003 John Michael Greer became the seventh Grand Archdruid of the Ancient Order of Druids in America (AODA), an initiatory organization teaching Celtic nature spirituality which was founded in 1912. The outcome was that his writings began to stray into territory very far from the Hermetic occult philosophy that had been the previous focus of his career. The essays included in this volume chronicle some of the themes he explored as a result: Druidry, Jungian psychology, politics, history, and the shape of the future in a society in decline.

Cahill explores the origin of civilization, using information from history, archaeology, mythology, linguistics, geology, astronomy, and philosophy to learn more about mankind.