Many principal supervisors in the United States work in relative isolation and experience no professional development designed to support them. Over the past four years, the role of principal supervisor in Long Beach Unified School District in California has shifted from compliance and at-a-distance supervision practices to a deeply rooted coaching orientation, support for addressing student achievement issues, and a focus on quality instruction in every classroom. To ensure that every principal receives support from a skilled principal supervisor equipped to coach effectively and provide targeted support, the district created a professional learning community (PLC) for principal supervisors. At the heart of the principal supervisor PLC is the district's lab day. Lab day gives principal supervisors the chance to practice blended coaching and their supervisory decision making in an authentic and active environment, using peer and expert feedback throughout the session. In addition to a focus on blended coaching, lab day gives principal supervisors time to collaborate and problem solve with other principal supervisors about their work with individual principals. Through case study analysis, principal supervisors share their successes and offer ideas to one another, often resulting in significant rethinking of their work with their supervisees. Long Beach has experimented with variations in the lab day model and shared the model with the 14 districts participating in The Wallace Foundation's Principal Supervisor Initiative, resulting in experimentation by 13 other school districts as part of the national Principal Supervisor Initiative professional learning community. This article provides an itinerary of Long Beach Unified School District's Principal Supervisor Lab Day. Based on the district's experience with the Principal Supervisor Lab Day, the authors offer suggestions for characteristics that should be present across all configurations of the lab day. Lab days and the principal supervisor community of practice are clearly having a positive impact on the culture among principal supervisors and in the relationship between principals and principal supervisors. Recent data show that, even as expectations for principal performance are rising through clearer and more consistent implementation of the principal evaluation rubric, principal performance is improving.