Before the Council of Trent, the plainchant office was the most widely cultivated musical expression in religious composition. However, its prominence has not yet been matched by a proportionate number of publications, primarily due to the limited knowledge of the sources and the lesser appeal of studying sacred monophony compared to polyphony. This conference aims to summarise the main findings of an extensive investigation conducted over several years, focusing on the Iberian Peninsula from the 12th to the 16th century. The characteristics of the repertoire will be illustrated at several levels: 1) Liturgical affiliation 2) Organisational framework; 3) Diffusion and authority; 4) Literary creativity; and 5) Musical peculiarities (notational demands and melodic and modal behaviour). This broad transnational exposition will reveal the specific characteristics of the different Iberian territories, with a particular focus on Portugal. Finally, the challenges involved in studying and editing this liturgical corpus will be reviewed in order to prepare several publications.