The church of St Andrew in Abay is the result of different construction phases, as tends to be the rule in the majority of religious buildings in the area. The original structure (Romanesque and possibly 11th century), was greatly altered in subsequent periods according to the artistic movements of each age. The original temple was a simple construction with a single nave, wooden roof structure and semicircular apse, austerity being the main characteristic of the Romanesque style.

A smaller nave was added to the original building on the south side and a tower built above it which connected with the main nave through a rounded arch. The outside of the main apse is decorated with some figures considered typical of the Romanesque of Jaca, which has led to speculation regarding the possible early origins of the church.

The majority of the rebuilding and modification work was carried out in the 16th century. The building was extended to three naves and a new entrance was designed consisting of a Classicist doorway with a rounded arch flanked by Tuscan pilasters. The doorway is inscribed with the date of completion of the work – 1575.

The last significant building work was carried out in the 18th century with modifications to the structure of the barrel vaults which have been conserved to the present day. The tower was extended to the three floors we see now. Also from that period are the rosettes, a decoration of popular character, in the apices of the vaulted ceiling.

The interior is of sober decoration. The choir gallery, built in the 16th century, is made of wood and is decorated with geometric designs also popular in other churches of the Jacetania area. A few examples of the liturgy of the first church have also been conserved along with some altarpieces and an interesting wooden carving of the Virgin seated with the baby Jesus on her lap.