This thesis explores mood selection in Old Italian, describing the use of the subjunctive and indicative in complement clauses in non-literary Tuscan of the Quattrocento (1375-1499). Using Wandruszka’s (1991) model of the subjunctive, and a Tuscan corpus of merchant letters and ricordi, sermons and other religious writing, based on Tavoni’s (1992) hierarchy of non-literary Quattrocento writings, I quantitatively assess the factors that influence mood selection in complement clauses. I restrict my analysis to complement clauses so as to compare the findings with those of Stefinlongo’s (1977) and Vegnaduzzo’s (2000) similar corpus-based studies of mood selection in thirteenth-century Italian, where possible, in order to suggest any trends in the use of the subjunctive. While I find that the semantics of the governing lexical element still has the predominant influence on the mood of the complement clause in fifteenth-century Italian, I also find that other factors, such as clause type, person and number, and tense and aspect, have a significant role in the modal outcome of complement clauses. However, the influence of these other factors is neither categorical nor equal, and it may be collective in the case of co-present factors. By conducting a quantitative comparison of mood selection in a variety of text types, my study also investigates Stefinlongo’s hypothesis that subjunctive use is not influenced solely by semantic or syntactic factors but also by features at the level of text type. However, I find the modal influence of text type to be largely indirect, influencing the relative incidence of different semantic contexts which in turn influences the incidence of subjunctive and indicative in a text. The findings of this study serve to inform our understanding of the evolution of the subjunctive in Italian.