This thesis examines the situation of the Latin language in the unique linguistic environment of early fifteenth century Florence. Florence, at this time, offers an interesting study because of the vernacular language's growing status in the wake of the literary success of vernacular authors Dante, Petrarch and Boccaccio, and the humanist study of Greek language. Joshua Fishman's theories on threatened languages and Reversing Language Shift are used to examine Latin's position in this environment. Chapter I describes Fishman's theories and applies them to the special situation of Florence, giving a context for the following three chapters. Chapter II offers an original interpretation of Leonardo Bruni's Dialogus ad Petrum Histrum, emphasising the significance of the speaker, Coluccio Salutati, and his apparent message in favour of reviving spoken Latin. Chapter III describes a debate that began in 1435, after the papal Curia moved to Florence and Bruni was drawn into the discussions of the papal humanists. The debate examined whether the Ancient Romans actually spoke Latin in their daily lives, or whether Latin was primarily a written, literary language, and there was a separate, spoken language for domestic environments, as in Florence in the fifteenth century. A number of humanists commented in response to this question. I examine Flavio Biondo's treatise dedicated to Leonardo Bruni, Bruni's letter in response to Biondo, Poggio Bracciolini in the the Tertiae Convivialis Historiae Disceptatio, and finally, Leon Battista Alberti's comment in the preface to the third book of the Della Famiglia. In Chapter IV, Bruni's vernacular writing, the Vita di Dante,is used to establish Bruni's own attitude to language choice as flexible and dependant on the subject matter, genre and intended audience for the work.