The purpose of this study was to find out which assessment data are used by elementary principals who are currently practicing in Title One California elementary schools with populations of at least 600 students, API scores of 700 or higher, and 40% or more English Learners (ELs) to inform instructional efforts at the school site level to improve EL academic performance in literacy, how the assessment data is being used, and what resources principals perceive are needed to support data-driven EL-related instructional improvement efforts. This study utilized a descriptive survey method for data collection. The researcher collected data from 42 elementary principals in person at the ACSA Leadership Summit Conference in Sacramento November 5-6, 2009. Principals completed a four-section questionnaire that included questions about school information, EL assessments, EL assessments and academic improvement, and instructional resources needed for EL academic improvement. The key findings were that most principal respondents stated that the most frequently used assessments for ELs were the Curriculum Based Measurement, the Houghton Mifflin Reading Skills Assessment, and ELD portfolios, all formative assessments which inform instruction, rather than the CELDT, an annual summative assessment. Principals reported using grade level team meetings to analyze the assessment data and plan lessons, frequently using assessment data to address the academic needs of the students, monitoring student progress in ELD using ELD curriculum assessments, and participating in one-on-one conferences with teachers as ways to use assessment information to improve EL academic performance in literacy. Instruction for ELs was differentiated daily during IWT or RTI. The resources identified by principals as most needed for EL academic improvement were professional development in how to differentiate instruction for English Language Development (ELD) levels followed by the need for professional development in how to use data to drive instruction. In effective schools with ELs, the leadership skills of the principal, using certain formative assessments, and analyzing the assessment data in grade level teams led to EL success. Twenty-four percent of the students in California public schools are ELs. In this study, principals have identified the best assessments to use with ELs to ensure high academic achievement. [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.]