This period of British history saw dramatic social, political, and cultural changes, characterized by the great movement of peoples. The Stone Age peoples, Bronze Age peoples, Celts, Scots, Picts, Irish, Roman, Anglo-Saxon, Danes and Normans all arrived, settled, and (to some degree) intermingled. Each of these peoples has a complex history partly separate and partly shared, sometimes obscure, sometimes distorted in the popular imagination, and the purpose of the encyclopedia is to both highlight specific details and clarify the overall picture.

The geographic scope of the encyclopedia is Britain and Ireland, and chronologically covers everything from the Neolithic period to 1154. A section of longer essays on key themes is followed by an A-Z section of shorter entries on specific topics. Entries vary in length from about 400 words to about 7,500 words. Each entry includes a brief bibliography. This encyclopedia will be a useful reference for nearly every level of research, from general background information on a select topic for the lay reader to the latest and best research and historiographic trends for advanced researchers