Principals used distributed leadership, as suggested by researchers, to actively engage teacher leaders in instructional leadership responsibilities. It was not known how high school principals' distributed leadership practices enabled teacher leaders to assist principals with instructional leadership responsibilities. The purpose of this qualitative multiple-case study was to understand how high school principals' distributed leadership practices enabled teacher leaders to assist principals with instructional leadership responsibilities at 12 high schools in Southern California. Two research questions guided this study: (a) How does the principal use distributed leadership practices to enable teacher leaders to assist principals with their instructional leadership responsibilities in high schools in Southern California? and (b) How have teacher leaders assisted principals with their instructional leadership responsibilities in high schools in Southern California? Spillane's model on distributed leadership practices provided the theoretical framework. The triangulated data from interviews, observations, and document analysis, confirmed that the study's findings provided evidence of the 12 high school principals' experiences: (a) principals' collaborative and coordinated efforts empower teacher leaders to co-lead the instructional program at their schools, and (b) teacher leaders are influential in assisting principals to lead and support the instructional program. The findings may be instrumental in ensuring principals' use distributed practices to enable teacher leaders to assist them with instruction-related activities. The results of this study may serve as a guide to aid principals in accomplishing their instructional leadership responsibilities. [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.]