The purpose of this study was to compare the science achievement and high school completion rates of students in a large high school implementing professional learning community concepts and practices with two large high schools not participating in professional learning community concepts and practices. The primary methodology employed was a causal-comparative quantitative study. Information regarding student achievement and professional learning community concepts and practices was collected. The data collected included: archived 2008 and 2009 Texas Academic Knowledge and Skills (TAKS) test scores obtained from Confidential Student Rosters provided by the Texas Education Agency, archived high school completion rate data obtained online from the Texas Education Agency's Academic Excellence Indicator System for 2008 and 2009; and survey responses from science teachers, administrators, science instructional facilitators and science department heads. The following conclusions were derived based on the data analysis in this study: (1) Professional learning community concepts and practices identified by DuFour et al. (2006) were being implemented in large high schools with 2,000 or more students with and without a formal implementation plan. (2) Large high schools with 2,000 or more students with identified professional learning community implementation plans have a higher level of implementation of concepts and practices identified by DuFour et al. (3) Professional learning community concepts and practices positively affect science student achievement in large high schools with 2,000 or more students. (4) The implementation of professional learning communities in large high schools with 2,000 or more students does not appear to have an impact on students' Commended performance on the science Texas Assessment of Knowledge and Skills (TAKS). (5) The high school completion rate for "all students" is higher for large high schools with 2,000 or more students implementing a formal professional learning communities plan than it is for large high schools not implementing a formal professional learning communities plan. [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.]