This quantitative study reviews the impact on student achievement following professional development on the principles of formative assessment. The study compared mathematics and reading performance data from student populations with teachers who received training in formative assessment to performance data from student populations with teachers who have not received training in formative assessment. The performance data included the Virginia Standards of Learning, as well as quarterly benchmarks and diagnostic assessments of students within Loudoun County Public Schools in Northern Virginia. The results of the findings lead to the rejection of the null hypothesis in 4 of the 7 research questions. Consequently, the study found that students who received instruction from teachers with training in formative assessments had higher scores in 3 key areas. The assessment data showed a statistically significant difference in growth in the area of mathematics on the state and diagnostic assessments. The present study also showed a statically significant difference in growth in the area of reading on the state assessment. Despite the mixed results, this study adds to previous research on teacher effectiveness resulting from high-quality professional development on the principles of formative assessment and the impact to student achievement. The implication of this study serve to validate that the utilization of the principles of formative assessment can help teachers and students reach an optimal level of teaching and learning. Appended are: (1) Pre/Post Survey Results; (2) Instructional Audit/Walkthrough Results; and (3) Loudoun County Public Schools Research Approval.