It faces onto the Piazza Navona, one of the main urban spaces in the historic centre of the city and the site where the Early Christian Saint Agnes was martyred in the ancient Stadium of Domitian.

As well as religious services, the church hosts regular classical concerts in the Borromini Sacristy, from sacred Baroque works to chamber music and operas.

Carlo Rainaldi was reappointed and made a number of modifications to Borromini's design including an additional storey to the flanking towers and simplifying their uppermost parts.

The cupola is frescoed with the Apotheosis of Saint Agnes, begun in 1670 by Ciro Ferri and finished after his death in 1689 by Sebastiano Corbellini.

The near-circular interior, actually a Greek cross design, is circumferentially surrounded by marble sculptural Baroque masterpieces, dedicated to individual martyred saints.

Among the sculptural decoration are the following: Inside the church is also a shrine for Saint Agnes, containing her skull and a marble relief by Alessandro Algardi.

It is often said that Bernini sculpted the figure of the "Nile" covering his eyes as if he thought the façade designed by his rival Borromini could crumble atop him.

Most prominently, during the Pamphili papacy, an official commission was established to study defects that had arisen in the foundations of the belltowers (built under Bernini's guidance) in the façade of Saint Peter's Basilica.