This thesis is a sociolinguistic study of Preterit and Present Perfect (PP) usage in contemporary and earlier Argentinian River Plate Spanish (ARPS). The data analyzed stem from a 244,034-word corpus collected for the purposes of the study, including contemporary casual conversation, sociolinguistic interviews, participant observation, written questionnaires, and newspapers spanning the 19th–21st centuries. / The study is motivated by previous claims that in Latin America the PP is restricted to contexts that extend into the present time, resembling Peninsular medieval and Renaissance usage (e.g. Lope Blanch 1972: 138; Harris 1982: 50; Squartini & Bertinetto 2000: 413). I challenge this proposal showing that (1) ARPS has undergone its own development, and (2) Latin American varieties do not represent earlier frozen developmental stages akin to earlier Peninsular Spanish. / Although low in overall frequency, the contemporary ARPS PP is used in experiential settings to express indefinite past (a vernacular use). Moreover, multivariate analysis of the contemporary oral data reveals that the ARPS PP is not aspectually restricted to repetitive and iterative contexts extending into speech time – contrary to Schwenter and Torres Cacoullos’ (2008) findings for contemporary oral Mexican Spanish. Indeed, the data show that the ARPS continuative PP is losing its link-to-present requirement. The ARPS PP also features minimally in resultative and continuative settings, supporting layering of old and new grammaticalizing structures (Bybee, Perkins & Pagliuca 1994: 21). Present relevance does not determine ARPS PP usage and is instead encoded through the Preterit and temporal adverbials (TAs). / Historically, the PP has dwindled in usage frequency since the 19th century and the Preterit has invaded the spaces erstwhile occupied by the PP. PP functions like result, continuity, current relevance, and hot news are currently fulfilled by the Preterit, in combination with TAs (TA + VERB-PRET). I argue that the TA + VERB-PRET construction has emerged as a periphrastic encoder of PP nuances, a development reminiscent of perfect periphrases in languages such as Yoruba and Karaboro (Niger-Congo) (Dahl 1985: 130). A contemporary example of this construction includes the widespread temporal marker ahí ‘at this point in time’ (lit. ‘there’) in combination with the Preterit to indicate temporal immediacy. / The contemporary ARPS PP is sociolinguistically constrained; men use it significantly more often than women. The PP is also employed by younger speakers, challenging the position that this form is on the verge of extinction (Kubarth 1992a: 565; Burgos 2004: 103). In contrast to the contention that the PP occurs more frequently in written media (e.g. De Kock 1989: 489; Squartini & Bertinetto 2000: 413), the contemporary oral and newspaper corpora show similar distributional tendencies. Only in the questionnaire is the PP used more readily in ways unattested in oral interaction (i.e. in current relevance and past perfective settings). ARPS ambivalent use of the PP represents the essence of the so-called “actuation problem”; that is, the contention that the process of linguistic change involves stimuli and constraints from both society and from the structure of language (Weinrech, Labov & Herzog 1968: 186).