Understanding and implementing effective classroom assessments are essential to increasing student achievement (Marzano, 2000). Teachers' assessment literacy is viewed as a key link in the connection between assessment quality and student achievement. According to Marzano, the relationship is seen as important as any other in the teaching and learning environment (p. 5, 2000); therefore, this study was designed to measure and describe the levels of assessment literacy, as measured by, "The Standards for Teacher Competence in the Educational Assessment of Students" (AFT, NCME, & NEA, 1990), for high school teachers within the Central Bucks School District. The primary goal of this study was to examine the level of assessment literacy to which high school teachers within the Central Bucks School District have regarding the development and implementation of effective classroom assessments. This study gave voice to high school teachers by facilitating their participation in a questionnaire and interview which has the capacity to impact the method by which staff developers and school administrators approach future teacher professional development. A mixed method, explanatory design approach involving two phases set a priority on phase one quantitative data collection aimed at identifying a population of 180 Central Bucks School District core-subject area high school teachers' level of assessment literacy by utilizing the questionnaire "Assessment Literacy Inventory" (Mertler, 2002). Results stemming from phase two one-to-one interviews of a purposeful sample population served as qualitative data and increased the reliability and applicability of explaining quantitative results as well as collecting teachers' perceptions related to skills associated with being effective classroom assessors. This research study was planned and carried out in order to meet ethical acceptability. In planning and conducting this research, as well as in reporting all research findings, the researcher fulfilled two obligations in order to meet ethical standards set forth by the American Psychological Association. First, the research project was planned so that the chance for misleading results was minimized. Secondly, steps were taken to protect and ensure the dignity and welfare of all participants (APA, 2010). [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]