The appearance of the edifice is mostly defined by its northern side, originally parallel to now disappeared walls. The southern side faces the cloister while the apse side and the façade are barely visible due to the nearby edifices. The northern side has received in fact most of the external decorations, including a rich portal with a sculpted architrave, signed and dated (1166) by the master Gruamonte and depicting the "Last Supper". The pattern of the wall is typical of other buildings in Pistoia, and inspired to the contemporary Pisan Romanesque: it features rows of small arcades on small or blind columns with small windows and lozenges inscribed within the arches. The stones used, white and green in color, are respectively marble and serpentine from Prato.
During the last medieval enlargement, the church received its current plan with a single hall and a rectangular apse, incorporating the former northern wing of the cloister. What remains of the latter, dating to the 12th century, is today the only example in Pistoia of a Romanesque structure in mixed stone and brickwork construction. The small columns are in stone, decorated with capitals featuring heads of lions and oxen while the arches and the walls are in brickwork. In the 14th century, it received a second floor with a loggia.
|Left of the entrance, on the northern wall, is a white ceramic glaze depicting the "Visitation", by Luca della Robbia. It is the oldest surviving example of the use of this technique in his workshop, aside from friezes or bas-reliefs. The work, originally featuring gilt decorations on the hair and the clothes, was commissioned in 1445 by the Fioravanti family of Pistoia. It was probably located then on the side opposite the present one.
From the 12th-13th century is the holy water font in the mid of the nave, attributed (in the upper part) to Giovanni Pisano. It depicts the Cardinal Virtues, supported by caryatids of the three Theological Virtues, attributed to a pupil of Nicola Pisano.
On the southern walls is the ambon of fra Guglielmo da Pisa, for which it has been supposed a collaboration by Arnolfo di Cambio. Executed in 1270, it was initially located in the Romanesque presbytery and moved to its present position in 1778. The high-relief sculptures, in Apuan marble Realizzato nel 1270, had originally a polychrome glass background, now mostly lost. At the steps of the columns are sculptures of a lion.
In the presbytery is the polyptych by Taddeo Gaddi (1350–1353), depicting the Virgin with Child with the Saints James, John the Evangelist, Peter and John the Baptist. Over the main figures, inscribed within Gothic-style small arches and twisting columns, are other figures of saints; in the upper frame is an Annunciation within a mullioned window, surmounted by the Eternal Father.
From 1307 are the frescoes in the choir, with stories of the History of the Passion, attributed to the Master of 1310. The church houses also a 13th-century crucifix.
Holy water font, by Giovanni Pisano (ca 1248-ca 1315).