Research that examines diachronic change and modality posit that modal verbs follow certain universal paths of development (e.g. Cornillie, 2007; Bybee & Fleischman, 1995; Bybee, Perkins & Pagliuca, 1994). The present study examines the development of Spanish modality in Mexico through the use of multivariate analyses, relative frequencies, and the comparative method as a means to uncover where in the grammar changes are taking place. In addition, this study exposes the subtle semantic differences of four obligation markers--"haber de," "tener de," "tener que," "hay que"--commonly used in Mexico between the 16th and 21st centuries. Results from this study aim to explain the use of one marker over another by operationalizing various factors found to contribute to changes in modal systems across languages. The factors examined include: type of verb, animacy/grammatical person, type of sentence, tense-aspect-mood, temporal markers, polarity, objects (direct, indirect), preceding clitic, lexical verb type, sex and age. Working within the usage-based framework, 5691 tokens were extracted, analyzed and compared across centuries using relative frequencies and the statistical program GoldVarb (Cedergren and Sankoff, 1974). Results indicated that there is an overall shift in preference from "haber de" to "tener que," suggesting that the system is undergoing longitudinal renewal. This is supported by the dramatic increase in frequency of "tener que" between the 19th and 20th centuries and the sharp decrease of "haber de" during this same period. A statistical examination of this shift indicates that change is occurring within grammatical person/animacy. Research on modality (e.g. Bybee and Fleischman, 1995) suggests that, as obligation markers develop, they will occur more often in contexts with third-person subjects so it is not surprising that animacy/grammatical person may be an area in the grammar where change takes place. In addition to the outcome from the 19th and 20th centuries, the results also expose the various factors that condition the use of each of these obligation markers over time. Through evidence found in processes such as semantic weakening, decategorialization, the results corroborate postulations of a universal path of change in modality. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]