A striking feature of medieval Irish literature is the prominence accorded to beings who are sometimes called the 'Túatha Dé' (Tribes of the Gods), and sometimes the people of the síde or hollow hills: a magical race, in whom it is possible to recognise the divinities once worshipped by the pre-Christian Irish. Tales in which the Túatha Dé have a central rôle have conventionally been assigned by scholars to a ‘mythological cycle’. This book examines the background and rationale for this classification; discusses the nature of the Túatha Dé themselves; and offers a survey of the tales of the Túatha Dé that are attested from the Old Irish, Middle Irish and Early Modern Irish periods.