A continuous frieze, carved in low relief, completed the decoration of the Parthenon and at the same time constituted a structural feature of the building. It was 160 metres in length and ran above the colon-nade of the pronaos and the opisthodomos and the walls of the cella. The frieze blocks were laid in place on the building between 442 and 438 BC and probably carved in position. The theme of the frieze is thought to have been inspired by an actual event, the procession of the Panathenaia festival. The Great Panathenaia were celebrated every four years in honour of the goddess Athena and included various religious activities, as well as horse racing, music competitions etc. The culmination of the feast was the procession to the Acropolis and the offering of a new peplos (robe) to the votive statue of Athena Polias, on her birthday.

Two separate streams of figures started off from the southwest corner of the building and, moving towards the east side, met up above the entrance of the temple, where the peplos was delivered in the presence of the Olympian gods. The west frieze, copies of which are exhibited here, depicts men and youths with their horses preparing for the procession. Some are fitting the bridles on their horses or adjusting their own garments, others are starting to mount and ride towards the northern side, where the procession has already started. Some are hoplites, as is indicated by their breastplate, helmet or sword. On the left hand side of the frieze a marshal (an official responsible for the flow of the procession) motions with his left arm and turns towards the first two approaching horsemen. A youth prepares his horse with the help of his young servant, in the presence of an official.
Subsequent blocks depict various scenes from the preparation of the procession, interposed with figures of one or two horsemen riding at a steady canter. The scene that decorates the middle block is exceptional: a mature rider trying to restrain his rearing horse. It is one of the most beautiful compositions of the frieze and is considered to be the work of Pheidias himself.

On the west frieze, from a total of 16 blocks, the sculptured surfaces of the first two from the left were cut out by Lord Elgin's workmen and, in the form of slabs, are exhibited now in the British Museum. In 1993, the remaining blocks were taken down from the monument and transferred to the Acropolis Museum for preservation, conservation and exhibition.