Spain and Portugal possess a wealth of medieval churches, including many with original furnishings. Most outstanding of all is the altarpiece or 'retablo', the epitome of Iberian ecclesiastical art. With its exceptional dimensions, architectural structure and extensive imagery, it became clearly distinct from altarpieces elsewhere in Europe. The retable cannot be understood simply as an artistic genre, but must be viewed in its spatial surroundings and against its religious and socio-cultural background. In the present study, the medieval altarpiece in the Iberian Peninsula is approached as a witness to liturgy, faith and devotion. Part I sketches its morphological development from its origins around 1100 to the end of the Gothic age in the first half of the sixteenth century. Part II analyses the retable in its spatial context, formed by the architecture of the church building and other elements of the interior such as the choir. In Part III the retable is discussed as a means of communication, conveying a message between the patron who commissioned it and the recipients.Both as an art object and as a bearer of imagery, the altar retable played an important part in the staging of liturgy in the medieval church of the Iberian Peninsula.