On the basis of fragmentary evidence from the ancient Celtic languages (e.g. Gaulish), but especially from the rich poetic heritage of the medieval Insular Celtic languages (e.g. Old Irish, Middle Welsh), the poetic terminology reconstructable for Common Celtic is presented. The possible metrical remains from ancient Celtic are reviewed and an attempt is made to identify their principles of versification. The characteristics of the medieval Irish and medieval British systems of versification are described. Finally, the question of the genetic relationship of ancient and medieval Celtic versification is discussed, and the possible relationship of Celtic with neighbouring Germanic, especially medieval Scandinavian, traditions is briefly reviewed.