The purpose of this relational, non-experimental, explanatory, cross sectional study with quantitative methods was to explain the influence of length of school day, if any, on Grade 4 and Grade 5 student achievement in Language Arts and Mathematics as measured by the high-stakes New Jersey standardized test entitled New Jersey Assessment of Skills and Knowledge (NJ ASK) 2011. Additionally, the study examined the influence of other student, staff, and school variables such as student mobility, student attendance, percentage of students eligible for free and reduced-price lunch (SES), percentage of students with limited English proficiency (LEP), percentage of students with disabilities, staff mobility, staff attendance, percentage of staff with master's degree or higher, and school size on the NJ ASK 4 and 5 in LAL and Math. The target variable of interest, length of school day, was found not to be a statistically significant predictor of achievement on the NJ ASK 4 or 5 in Language Arts or Mathematics. The results of this study indicate that no statistically significant relationship exists between length of school day and proficiency percentages on the NJ ASK 4 and 5 in LAL and Math. Of the variables included in this study, percentage of students eligible for free and reduced lunch (SES), student attendance, percentage of students with disabilities, and percentage of staff with master's degree or higher were found to be statistically significant predictors of student achievement in all eight regressions that were conducted. Additionally, school size and student mobility were found to be statistically significant predictors of student achievement when the dependent variable was NJ ASK Math, Grade 4 and Grade 5, respectively. [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.]