Santa Maria del Popolo
The architect or architects of this innovative project remain unknown due to a lack of contemporary sources. Giorgio Vasari in his "Lives" attributed all the important papal projects in Rome during Sixtus IV to a Florentine, Baccio Pontelli including the basilica and monastery of Santa Maria del Popolo. Modern researchers deemed this claim highly dubious and proposed other names among them Andrea Bregno, a Lombard sculptor and architect whose workshop certainly received important commissions in the basilica. The fundamental differences between the façade and the interior suggest that maybe more than one architect was working on the building.
Giacomo della Porta
Giacomo Della Porta was born in Genova from a family of sculptors. He was influenced by and collaborated with Michelangelo, and Giacomo Barozzi da Vignola, his teacher of architecture. With these two great masters he became one of the most important architects in the history of the Roman renaissance. In fact after 1563 he carried out Michelangelo's plans for the rebuilding of the Campidoglio or Capitoline Hill's open spaces where he completed the façade and steps of Palazzo Senatorio, and the "Cordonata" or the ramped steps up to the Piazza del Campidoglio.