The Basilica of St. Mary of the Angels and of the Martyrs (Latin: Beatissimae Virginis et omnium Angelorum et Martyrum, Italian: Santa Maria degli Angeli e dei Martiri) is a Catholic titular minor basilica and former Carthusian conventual church in Rome, Italy, constructed in the ruined frigidarium and tepidarium of the Roman Baths of Diocletian in the Piazza della Repubblica.

Monoliths of granite from Egypt, wood from the Bavarian forests held up the vaulted ceilings and arches, and intricate mosaics decorated the walls.

By a brief dated 27 July 1561, Pius IV ordered the church "built", to be dedicated to the Beatissimae Virgini et omnium Angelorum et Martyrum ("the Most Blessed Virgin of all the Angels and Martyrs").

Impetus for this dedication had been generated by the account of a purported mystical vision experienced in 1541 at Santa Maria di Loreto, Rome of the ruins of the Baths by a Sicilian monk, Antonio del Duca, who had been lobbying for decades for papal authorization of a more formal veneration of the Angels.

[2] At Santa Maria degli Angeli, Michelangelo achieved a sequence of shaped architectural spaces, developed from a Greek cross, with a dominant transept, with cubical chapels at each end, and the effect of a transverse nave.

Titled Light and Time, its abstract design functions as a sundial - by observing its reflection on the floor of the round vestibule, one can follow the movement of the sun across the sky.

[1] In April 2010, a five-metre-high (16 ft) bronze statue of Galileo Galilei Divine Man (designed by 1957 Nobel laureate Tsung-Dao Lee) was unveiled in a courtyard within the complex.

The church hosts the tombs of General Armando Diaz and Admiral Paolo Thaon di Revel, who were successful commanders during World War I on the Italian front.

At the beginning of the 18th century, Pope Clement XI commissioned the astronomer, mathematician, archaeologist, historian and philosopher Francesco Bianchini to build a meridian line, a sort of sundial, within the basilica.

[3] This church was chosen for several reasons: (1) Like other baths in Rome, the building was already naturally southerly oriented, so as to receive unobstructed exposure to the sun; (2) the height of the walls allowed for a long line to measure the sun's progress through the year more precisely; (3) the ancient walls had long since stopped settling into the ground, ensuring that carefully calibrated observational instruments set in them would not move out of place; and (4) because it was set in the former baths of Diocletian, it would symbolically represent a victory of the Christian calendar over the earlier pagan calendar.

In addition to using the line to measure the sun's meridian crossing, Bianchini also used the window behind the pope's coat of arms and a movable telescope to observe the passage of several stars such as Arcturus and Sirius to determine their right ascensions and declinations.