When children transition from Head Start to kindergarten, they may face different environments and expectations, which can cause disequilibrium and challenges for young children who benefit from routines. These kindergarten transitions occur within a broader context of distinctly different systems, with differing and sometimes contradictory governance structures, policies, philosophies, and accountability metrics. To date, much of what is known about supporting children’s transition to kindergarten narrowly attends to the classroom-level practices implemented by ECE programs and kindergarten teachers separately. There has been limited attention on the multi-layered perspectives, policies, professional supports, and practices—what we call “the 4Ps”—within and across the ECE and K-12 systems that are also needed to adequately support these transitions. The HS2K Case Study was designed to help address gaps in the literature and further explore and refine the HS2K theory of change that was informed by literature and practice knowledge. This study is one of the first to focus on the kindergarten transitions, professional supports, and practices from multiple perspectives across the two systems that influence transitions. Through this study, we aim to provide holistic and rich descriptions of the strategies, relationships, and practices supporting kindergarten transitions across selected partnerships between Head Start and K-12 entities. In doing so, we aim to better understand the complex ecology influencing the experiences of children and their families. (author abstract)