The usage and disappearance of the Central Ibero-Romance future subjunctive have been extensively researched through Old Spanish texts. Studies on the future subjunctive as it evolved in the farther Western Ibero-Romance languages, represented by Galician and Portuguese, have been scarce, if not incomplete. This dissertation partially fills the gap in research on the development of the future subjunctive in Galician-Portuguese. It examines the morphological properties of the future subjunctive and investigates its syntactic distribution through the examination of two 13th-century texts: "Cantigas de Santa Maria" "Canticles of Holy Mary" and "A demanda do Santo Graal" "The Quest of the Holy Grail." The verb forms collected through careful examination of these texts were analyzed and projected in graphs and tables. Upon comparison, differences in the usage of clause types in these 13th-century texts seem to reflect variations in their thematic structures. The future subjunctive verb forms appear recorded separately in alphabetical order. In appendices, two lists with the future subjunctive clauses appear within the context of their main verb tenses in order of occurrence. Also listed are the conjunctions and relative pronouns characteristically preceding the future subjunctive clauses. The center of the discussion develops as the future subjunctive survives in Modern Portuguese as a function of a tense of discourse. High discourse frequency of salient (special) verb forms contributed to the preservation of the future subjunctive, averting morpho-semantic merger with homophonous forms of the personal infinitive. Examples culled from selected texts document the morphologic changes during the medieval period. A brief historical overview suggests that a direct descent of the future subjunctive from a Latin tense is improbable. Instead, the collateral or adjunct status of the future subjunctive, based on the conflict of its semantic features with a governing verb in sentential complements, forms further evidence that it is an innovation which arose from changes discernible in the tense system of spoken or Vulgar Latin, some of which already had Classical Latin antecedents. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]