This dissertation examines the extent to which lexical aspect and grounding influence the acquisition of tense and aspect by ninety-four instructed adult learners of Italian in two academic settings in North America. The semantic notions of tense and aspect have been extensively investigated in the domain of second language acquisition. After more than two decades of inquiry, conflicting views and a plethora of hypotheses and theoretical explanations have been proposed to account for the complex phenomenon of tense and aspect acquisition by second language learners. This study on past tense marking in L2 Italian is framed within two hypotheses: the Aspect Hypothesis and the Discourse Hypothesis. The rationale for selecting these two theoretical constructs rests on the assumption that the two categories of tense and aspect may interact in the encoding of temporal expressive devices and narrative functions. Data consisted of a grammaticality judgment task (GJT), a sentence completion task (SCT) and a narrative elicitation task. The ninety-four participants were selected according to three levels of proficiency: beginning, intermediate and advanced. Specific aspectual categories of perfective and imperfective predicates were analyzed by means of a repeated measures ANOVA model. Results from the GJT and SCT showed significant main effects for lexical aspect and interaction effects for level and aspect. In contrast, grounding overrides aspect in the selection of past tense markers in the oral narrative task.