Morales Mass Book is an open-access companion site to the First Book of Masses by Cristóbal de Morales (ca. 1550-1553), a Spanish composer at the Papal chapel in Rome. Our site explores the Missarum liber primus (Lyon: Moderne, 1546) focusing on the composer, the printer, and the processes that informed the composition of four of Morales’s polyphonic masses. In addition, video and audio recordings invite us into the world of Morales’s superb music. The physical 1546 edition of the Missarum liber primus was acquired by the John J. Burns Library in 2011.