Public education in the United States has become increasingly focused on school accountability measured by student performance on standardized assessments. Thus, monitoring students' learning progress throughout the year by using formative and summative assessments has become a priority for many schools and districts. Although teachers are often expected to create their own assessments for classroom or grade-level use, many teachers lack the training and preparation to effectively develop, administer, and interpret the outcomes of assessments. Furthermore, school leaders also lack understanding of effective assessment practices and are not equipped to support teachers in the development and instructional use of high-quality assessments. Research suggests there is a need for professional learning to impact assistant principals' ability to lead and monitor the development of rigorous, standards-based assessments that accurately measure student learning and can be used to guide future instruction. This action research case study sought to understand how a professional learning course in assessment literacy impacted assistant principals' overall assessment literacy and leadership self-efficacy. The following research questions guided the study: 1) What do assistant principals learn from participating in assessment literacy professional learning? 2) How do assistant principals describe the impact of assessment literacy professional learning on their leadership self-efficacy? 3) What does an action research team learn from creating and implementing a professional learning plan for assessment literacy? To address the research questions, an action research team collaborated to plan and facilitate a professional learning course of study in assessment literacy designed for assistant principals. Data were collected and analyzed from questionnaires, a focus group interview, and action research team meeting notes. Conclusions about this action research study indicated that effective assessment literacy professional learning can positively impact assistant principals' leadership self-efficacy. This work offers implications and recommendations for school districts seeking to increase the assessment literacy and instructional leadership capacity of assistant principals. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]