This dissertation investigates the morphological and morphosyntactic processing of case-marking by native and nonnative speakers of Turkish, through behavioral and electrophysiological responses. The study explores the locus of case processing costs during first (L1) and second language (L2) word recognition both in isolation and in sentences. It identifies the factors leading to persistent problems that late L2 learners face in attaining native-like processing of case assignment. To this end, the first experiment (a visual lexical decision task) examines whether different case forms generate differential processing costs, based on four main comparisons that reflect case properties and its status in the inflectional paradigm: 1) structural (genitive, accusative) vs. lexical (dative) case; 2) argument (accusative, dative) vs. non-argument (genitive); 3) higher (genitive) vs. lower type frequency (accusative, dative), and 4) citation form (nominative) vs. oblique cases (genitive, accusative, dative). The behavioral findings show significantly larger processing costs (i.e., longer reaction times and lower accuracy rates) for the genitive than the nominative case (citation form) across both subject groups, and than other oblique cases in L2 group only. ERP findings show significantly larger processing costs for the genitive than the accusative, and for the dative than the accusative only in L2 group. When the same case-inflected nouns were placed in a sentence context, larger N400 effects were found for the genitive, compared to the nominative and accusative in L1 group only. Together, these results suggest that different case forms generate differential processing costs in both subject groups, and L2 learners’ difficulty with the non-argument genitive and lexical dative oblique cases are at the level of form rather than sentence structure. The second (sentence) experiment also examined the processing of case errors (i.e., substitution of the accusative for the dative or vice versa on the object). ERP findings show a qualitative difference between L1 and L2 morphosyntactic patterns: P600 was missing while early negativities (N400 and left anterior negativity, LAN) were present in L2 group. These results suggest that advanced L2 learners evaluate the verb argument structure (LAN) and semantic fit (N400), but do not attempt to reparse the sentence (P600), unlike native speakers.