Clinicians, educators, and researchers alike continue to struggle without adequate and functional tools to measure language proficiency in bilingual populations. Language proficiency refers to the ability of an individual to use a language. However, the ways in which proficiency is classified are inconsistent and potentially invalid. Proficiency in young bilingual children is often determined through indirect measures (e.g., parent report) with unknown or inconsistent validity, impacting the field in both clinical and research arenas. The purpose of this study was to develop and validate a novel task that will allow us to quantify a child's functional language proficiency, while also identifying areas of language strengths and weakness across languages in Spanish-English 1st, 2nd, and 3rd grade children. The task capitalizes on the theory of natural translation (Harris & Sherwood, 1978), which refers to translation done in everyday circumstances by those who have had no special training. We evaluated task components and total task reliability and validity using test theory procedures. This work will set the foundation for quantifying and characterizing language proficiency in typically-developing Spanish-English speaking children. [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.]