I begin this study by discussing the architectural holdings of the Ottoboni family in Venice and Rome.I chronicle the projects of Cardinal Ottoboni in his official residence of the Cancelleria as Vice-Chancellor of the Church, and in the basilica of San Lorenzo in Damaso enclosed within the palace grounds.I characterize and suggest locations for his several palace theaters by assembling data never previously considered.For the first time, three permanent theaters are identified in his palace, the initial space by Simmone Felice del Lino on the ground floor as a commercial venture.I locate and reconstruct the cardinal's lost theater from Filippo Juvarra's drawings, room measurements, and palimpsests of decorations and architectural details in the palace.The findings are based on extensive documentation from Ottoboni family archives in the Vatican and Lateran holdings, the diary accounts of Francesco Chracas and Francesco Valesio, and the Correspondances of the French Academy in Rome.Ottoboni's projects for the basilica of San Lorenzo in Damaso included chapels by Sassi and by Gregorini, and over the years numerous grand devotional machine by most of his architects.His architectural commissions, both permanent and ephemeral, were almost exclusively official and public.The cardinal's participation in the competition for the façade of St. John Lateran in the early 1730s was the result of his function as the basilica's archpriest.His voice was but one of several in the final decision, causing him gradually to lose interest in the process.