This thesis is concerned the acquisition of person, as a formal feature of Brazilain Portuguese (BP), by normally developing and language- impaired (SLI) children. The working hypothesis is that children initially identify grammatical information concerning person in the closed classes delimited at the phonetic interface and that the locus of the interpretability of this formal feature at the semantic interface constitutes a difficulty children face in the acquisition of BP. The theoretical framework is the Minimalist Program (Chomsky, 1995), in the context of a psycholinguistic approach to language acquisition (Corrêa, 2002; 2006). The computation of Agree in the valuation of the person feature in T is considered both in the production and in the comprehension of linguistic utterances. The morphophonological expression of person is analyzed in the longitudinal data of the spontaneous speech of two Brazilian children (18 to 30 month old). Five comprehension experiments are reported, which aimed at investigating how person information is processed by 3 and 5 year olds (total= 131), from two social groups, as well as by two SLI children. The results suggest that the morphophonological expression of person both in D and in the verbal affix is present in children´s early production though morphological evidence of subject-verb agreement can be detected only by the age of 24 months.. The establishing of reference exclusively on the basis of the morphological expression of person presents difficulties for the normally developing children under five. SLI children older than five have particular difficulty in the comprehension of the third person and in the processing of both person and plural number information.