Variability in subject expression has been a widely studied phenomenon over the last few decades and is still the focus of a considerable body of research in both native (L1) and second language (L2) grammars. Crucially, the production of L2 Spanish learners, both written and oral, has been investigated in depth with a view to understand how they use referential expressions (REs) like null and overt pronominals (i.e. what has been traditionally called anaphora resolution) and other REs such as lexical noun phrases (NPs), as well as which factors constrain their use in real discourse (e.g. Blackwell & Quesada, 2012; Lozano, 2009b, 2016). Even though L2 learners acquire the morphosyntactic features that license null subjects in L2 Spanish from very early stages (Liceras, 1989; Phinney, 1987), results from both experimental and corpus-based developmental studies (e.g. Lozano, 2009b, 2018; Montrul & Rodríguez-Louro, 2006) have shown that certain features are particularly difficult for non-native speakers even at end-states of acquisition. L2 learners show persistent deficits in selecting felicitous null/overt pronouns when constrained at the interfaces (e.g. syntax–discourse interface), following Sorace’s Interface Hypothesis (2011, 2012), which holds that such features are more difficult to acquire than merely syntactic ones. However, Lozano (2009b, 2016) used a near-native corpus of L2 Spanish learners to show that these deficits are rather selective and do not necessarily affect the whole pronominal paradigm: most of these deficits were (i) attributed to third person human singular subject REs (whereas the rest of the pronominal paradigm was unproblematic), and (ii) were mainly observable in topic continuity scenarios (whereas topic shift and other scenarios were not problematic). These scenarios will be further explored in this chapter using a corpus approach, which will also allow for the investigation of other less-explored factors that constrain the form of subject REs in native and non-native grammars.