This paper discusses the grammatical and modular nature of functional categories by investigating different child populations. Children with only a right hemisphere make more errors on I than they do on D and C, suggesting that D and C are less lateralized and therefore less modular. In addition, the investigation of subjects (instantiations of I) in the Hebrew of an early Hebrew/English-speaking child shows that cross-linguistic influence from English to Hebrew takes place when it concerns subject realization, but not when it concerns subject-verb agreement, indicating that only part of I, namely Agr(eement), is in the core of grammar, and therefore less vulnerable. Finally, data from SLI shows that only Agr(eement) and overt article realization (instantiation of D) are impaired, but not subject realization (I) or article choice (D), suggesting that certain properties of I and D do not belong in the grammar. Thus, none of the traditional functional categories I, D, and C can be considered purely grammatical and are therefore less modular than originally thought. It is speculated that Agr(eement) is the only functional category with only grammatical properties, if it is a functional category at all.