In order to understand the nature of a given linguistic phenomena in the adult grammar, language acquisition research explores how children’s competence with respect to such a phenomena develops. However, diagnosing competence can be challenging because it is not directly observable. Researchers only have access to performance, which is mediated by additional factors and is not a direct reflection of competence. In this dissertation, I explore a case study of children’s early syntactic knowledge. My in-depth analysis of Principle C at 30 months provides novel insights into diagnostics for underlying competence by utilizing two distinct methods of analysis. The first analysis explores alternative mechanisms that have been proposed to account for early Principle C effects. By comparing across multiple linguistic contexts, I show that Principle C knowledge is the only mechanism which can account for all observed performance. The second analysis explores the deployment processes that are required to implement competence in performance. I present a novel analytic approach to identifying underlying knowledge which utilizes independent measures of these deployment processes. I show that individual differences in syntactic processing predict individual differences in interpretation, implicating syntactic processing in Principle C performance at 30 months. Together, these findings extend our knowledge of the developmental pattern that characterizes Principle C, which can contribute to debates about the origin of this constraint as part of the grammar. This research provides new depth to investigations of children’s early syntactic knowledge by highlighting new methods for diagnosing competence from observed performance.