This book presents an English-language translation of Risala-yi Mu?iniya, or the Mu?iniya Epistle. Risala-yi Mu?iniya is one of the earliest known works of Na?ir al-Din ?usi (1201–1274), an intellectual luminary of the 13th century CE. The work is notable for the choice of T?u¯si’s native Persian as the language of the text. In addition, T?u¯si organized his volume into a four-part structure, which went on to become a popular template for the Islamic astronomers who succeeded him.  This book helped ensure the patronage of T?u¯si's courtly patrons during his decades-long stay with the Isma?ilis, as well as the continuation of his remarkable career under the first Ilkhanid rulers of Persia. This translation helps make this notable treatise accessible to English language readers. It is among a handful of English translations of major astronomical works dealing with hay’a/cosmography in the Islamic world. Subsequently T?u¯si was to pen hisown commentary on the work (the ?all-i Mushkilat-i Mu?iniya, or A solution to the difficulties of the Mu?iniya) and he used this occasion to discuss his celebrated mathematical formulation “the T?u¯si Couple” (a concept that he merely hinted at in the Risala-yi Mu?iniya).