This dissertation explores parallels between Complementizer Phrase (CP) and Determiner Phrase (DP) semantics, syntax, and morphology--including similarities in case-assignment, subject-verb and possessor-possessum agreement, subject and possessor semantics, and overall syntactic structure--in first language acquisition. Applying theoretical research into CP-DP parallels to child language acquisition, it asks two questions: 1) is there a relationship between the acquisition of case assignment and agreement within the CP and the DP? and 2) does the appearance of a particular feature or structural position in the CP predict its appearance in the DP? Child language acquisition research has led to different conclusions regarding both the type of representations in child grammar and the nature of their grammatical development. Assuming a feature-based grammatical architecture without pre-defined functional categories, the dissertation evaluates these different approaches to whether and how the grammar changes. The dissertation describes the relevant morphosyntax of Estonian, Hungarian, and English, highlighting the DP-CP parallels and cross-linguistic similarities. Longitudinal data from three children for each language were analyzed, with attention paid to features shared between nominal and verbal domains or indicative of syntactic structure. Results relating to both the main research questions and grammatical development of each child were compared across the three languages. The study found that for Estonian, CPs and DPs did develop in parallel, with similar syntactic pairs being acquired simultaneously. Case, agreement, and person/number features associated with CP preceded DP counterparts. Hungarian was similar: syntactic development occurred in parallel, though morphological aspects, including possessor agreement, were acquired first in verbal environments. English results differed, with syntactic development again parallel, though case acquisition occurred simultaneously and morphology consistently preferred nominal realization. The results indicate a parallel and steady syntactic development, contrasted with morphological development influenced by frequency. Additional analysis showed that fewer features represented by a form led to earlier acquisition, with bound functional morphemes preceding free ones. Overall, this supports a weakly continuous view of grammatical development, with a steadily growing linguistic capacity limited by the need to learn particular linguistic features. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]