This MA Thesis aims to investigate the impact of assessment on students' overall English language proficiency at Mehmet Akif College, case of 10th-grade students. The topic was chosen because of the importance of assessment on the learning and teaching process, on students' achievement and performance. The main purpose of this study is to highlight the need of using a combination of different forms and strategies of assessment that yield students' progress and achievement. The research took place at Mehmet Akif College, in Lipjan. The subjects of this study were 10th-grade students, as well as English Language teachers involved in the teaching process at this school. The research is both qualitative and quantitative in its nature, using both questionnaires and interviews. This study's results indicated that there are different combinations of assessment use that enable enhancing students' proficiency; formative assessment improves the students' overall proficiency in the English Language and that summative assessment is effective in keeping track of student's achievement throughout the course.