Grandmother of all baroque churches, this huge structure was designed by the architect Vignola (1507-73) to be the tangible symbol of Jesuits, a major force in the Counter-Reformation in Europe. It remained unadorned for about 100 years, but when it finally was decorated, no expense was spared. Its interior drips with gold and lapis lazuli, gold and precious marbles, gold and more gold -- all covered by a fantastically painted ceiling by Baciccia (1639-1709) that seems to merge with the painted stucco figures at its base. St. Ignatius's apartments, reached from the side entrance of the church, are also worth a visit (afternoons only) for the trompe-l'oeil frescoes and relics of the saint.