The present dissertation is concerned with the use and interpretation of null and pronominal subjects in Brazilian Portuguese. This investigation examines these phenomena in an attempt to disentangle the semantic and discursive factors that can be relevant for choice between these anaphoric expressions in Brazilian Portuguese and the way in which this choice is articulated with the general theory of anaphora resolution. The starting point of this dissertation was the research looking into null and overt subjects from the perspective of Generative Grammar, specially the Parametric Theory. Throughout the present work, however, the analyses proposed in this perspective were shown not to account for the data at stake. The generalization that poor verbal morphology is directly related to the absence or reduced frequency of null subjects, for example, is challenged through experimental data and an investigation of the relative frequency of null subjects across discourse persons in corpora. An alternative explanation presented in the previous literature, namely the importance of the antecedents’ features of Animacy and Specificity, seems to better account for the attested distribution. However, this explanation is not sufficient for understanding the choice between null and overt subjects in Brazilian Portuguese, since the number of animate and specific null subjects is still relatively higher than in languages with obligatory expression of subjects. Therefore, it is argued that discourse factors seem to play a crucial role in the use of null and overt subjects in Brazilian Portuguese. The main factors identified here are Obviousness and Contrast. The first is a standard feature in the literature about anaphora resolution (expressed by a variety of terms, such as Salience, Familiarity, Accessibility, etc.), which is part of the reverse mapping hypothesis according to which the more obvious the subject is, the less explicit the co-referential form is allowed to be. The second factor, Contrast, is the main finding of the present dissertation: as is the case for other levels of linguistic analyses and other phenomena in language, the choice of anaphoric expression in Brazilian Portuguese seems to be driven by efficiency. In the present case, this means that, when the backgrounded information and the asserted (focused) in- formation in an utterance contrast the most, it is more likely that a null subject will be used. The design of a grammar that deals with these multiple features is sketched, specifically, a multi-layered scalar probabilistic grammar is proposed, whose semantic and discourse constraints act in parallel through a probabilistic mapping. It is, thus, shown that null subjects are likely in discursive co- reference, since in these contexts their antecedents are more obvious and the focused information contrasts the most with the background. An apparent counter-example to the proposal sketched here is analyzed: the generic interpretation of null subjects. However, it is shown that the same semantic constraints cross-linguistically applied to other generic constructions can produce generic null subjects in Brazilian Portuguese, given the failure to be grounded predicted by the approach proposed here. Finally, on-line evidence for the analysis of the use and interpretation of null and pronominal subjects is provided. The results found in three eye-tracking while reading experiments provide striking evidence in favor of the proposal put forward here, according to which null and overt subjects and their interpretation can be accounted for in terms of constraints on interpretation rather than licensing.