Unitary cardinals are a common source for indefinite markers. This thesis is a quantitative diachronic study of the development of Spanish un, from its cardinal value to its use as an indefi nite article. Based on a corpus comprising texts from the thirteenth to the seventeenth century, I present an analysis and chronology of the main changes undergone by un throughout this period, notably its increasing use as a marker of non-speci c indefinites, and its further incorporation in generic noun phrases and predicates. Additionally, I demonstrate that the development of the plural indefinite determiner unos is, with a few restrictions, parallel to that of its singular counterpart, not only in its increasing frequency, but also in its introduction into new contexts. Furthermore, I present a comparison between un and alg un in terms of speci city and conclude that although there are evident links between them, both being inde nite determiners derived from Latin unus, they have always had di erent functional domains. Finally, I show that one of the consequences of the incorporation of un into generic contexts is the rise of the so-called impersonal uno, and explain that this event is crucial to explain the disappearance of another generic pronoun, omne, whose last examples are found in the sixteenth century, that is, precisely the moment where the first instances of impersonal uno occur.