In Spanish, it is most commonly claimed that constituents in narrow presentational focus appear rightmost, where they also get main stress (1a), while stress in situ (1b) is infelicitous. (1) [Context: Who bought a car?]. a. Compró un carro mi [mamá][subscript F]. bought a car my mom. b. Mi [mamá ][subscript F] compró un carro. However, some recent evidence challenges this view, claiming that stress in situ (1b) is a possible strategy for marking focus in Spanish. This dissertation contributes new experimental evidence to this debate. Additionally, because focus involves the complex interplay of prosody, syntax, and discourse context, it is especially of interest when considering bilingual individuals. The grammars of heritage speakers of Spanish (that is, U.S.-born English-dominant bilinguals) are significantly different in a number of ways from those of Spanish monolinguals. One way they have been shown to differ is in phenomena regulated by the interfaces of syntax with other linguistic systems, i.e., precisely phenomena like presentational focus. We might thus expect that monolinguals and bilinguals would realize focus differently, as with other interface phenomena, and this dissertation brings experimental evidence to bear on this question as well. This dissertation thus has the dual motivation of investigating both presentational focus in Spanish and heritage grammars. It proposes an analysis of focus in Spanish in terms of conflicting constraints on well-formedness, using Optimality Theory, and then tests this analysis experimentally. The experiment consists of a contextualized aural acceptability judgment task, in which both monolinguals and heritage speakers listened to sentences in context and judged their discourse appropriateness. The main findings of the experiment were (i) both heritage speakers and monolinguals use stress shift (1b) to realize presentational focus, and (ii) monolinguals and heritage speakers did not differ from one another. The first finding runs contra the consensus in the literature and thus contributes to the growing challenge to this view, indicating that some common approaches to focus in Spanish may need to be rethought. The second finding was also counter expectations, and thus contributes evidence toward a more fine-grained understanding of heritage grammars with regard to interface phenomena. The results of this study are relevant to future studies of focus and other information-structural phenomena, as well as to future studies of heritage grammars and language contact, and it contributes new experimental data to both fields. [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.]