These letters were written between 1678-1687 by some of the first Spanish missionaries to the Mariana Islands. Seven are to María de Guadalupe de Lencastre, the Duquesa de Aveiro thanking her for support and aid in the mission. There is also one letter to Francisco García, a Jesuit priest in Madrid. Among the details included in the letters are descriptions of the European arrivals on the islands; Spanish and indigenous encounters; the daily life of Jesuit missionary work; reports of Jesuit deaths; ship and supply status; and requests for additional materials or men. Also included is a list of the Marianas Islands, divided into ones the missionaries had been able to reach and those they had not.