The goal of this dissertation is to address several major empirical and theoretical issues related to English-speaking children's difficulties with auxiliary use and inversion in questions. The empirical data on English question acquisition are inconsistent due to differences in methods and techniques used. A range of proposals about the source of auxiliary omission and inversion errors have been presented but they remain with a limited cross-linguistic scope and cannot explain the data in full. This dissertation addresses the issues by investigating the influence of various target-specific properties on children's production of "wh"- and "yes/no" questions in a cross-linguistic perspective. We propose a Hypothesis Testing model of language acquisition in which target specific properties of the language determine the types of errors and how difficult they are to correct. The more contrasts the language supports, the greater the number of incorrect hypotheses the child can entertain, and the longer it takes for children to acquire the target. Cross-linguistic variation in language acquisition is determined by differences in the amount of relevant contrasts supported in various target grammars. Languages with uniform properties facilitate the hypothesis-testing process by providing a narrow range of hypotheses to be considered and are learned easily. Languages with non-uniform properties extend the course of acquisition by providing a broad range of hypotheses to be tested. To test the proposed hypothesis testing model we conducted four elicited imitation experiments investigating 2-year-old children's auxiliary use and inversion in Bulgarian and English "wh"- and "yes/no" questions. We predicted similar performance on auxiliary use for the two language groups because English and Bulgarian are both inconsistent and support relevant contrasts regarding auxiliary inclusion. With respect to auxiliary inversion, Bulgarian is consistent and English is not. English supports three relevant contrasts that Bulgarian does not. Thus, we predicted that Bulgarian-speaking children would perform better on auxiliary/verb inversion than English-speaking children. Our results support the Hypothesis Testing model. [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/en-US/products/dissertations/individuals.shtml.]