Assistant principals exist in most school contexts and, although instructional leadership has been a frequent topic of educational research, most studies have focused specifically on the role of principals. The present improvement initiative analyzed the instructional leadership capacity of assistant principals and the impact of targeted professional development on assistant principals' instructional leadership capacity. Assistant principals were asked to participate in a 6-week, all-virtual professional development series emphasizing high-impact instructional strategies based on John Hattie's (2009) work. Assistant principal participant feedback and survey data were analyzed using a mixed-methods approach. Findings indicated assistant principals view instructional leadership as an important function of their role yet struggle to find time to address the instructional needs associated with that role. Additionally, assistant principals experienced increased confidence and frequency of engagement in instructional leadership activities after participating in professional development focused on actionable learning that can be implemented immediately as part of their professional practice. This study provides guidance for school districts seeking to implement professional development activities, including how school districts can use existing expertise from district leaders to improve the instructional leadership capacity of their 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://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]