Empirical evidence that horizontal alignment of policies and practices from preschool through the early elementary grades sustains the effects of quality preschool and contributes to children's learning is scarce, as discussed above, but there are nevertheless good reasons to expect benefits to such alignment. Moreover, many districts and schools in California and throughout the country are working to strengthen preK-3 alignment. The purpose of this report is to document: (1) how key district and policy leaders in California conceptualize preK-3 alignment; (2) what kinds of policies and practices districts have implemented to achieve alignment; (3) what kinds of efforts and practices have been successful and the critical ingredients of their success; and (4) the main barriers districts and schools have encountered when attempting to achieve horizontal alignment between preschool and the elementary grades. To achieve this goal, researchers conducted interviews with individuals who play key roles in early childhood education (ECE) in California, including foundation representatives and state-, county-, and district-level personnel. This report presents a qualitative analysis of the interviews identifying trends in responses, along with two district case-studies. The interview questions were designed to address six topics: (1) definition and key components; (2) value; (3) practices; (4) best practices, barriers, and need for change; (5) transitional kindergarten (TK); and (6) local control. Participants defined PreK-3 alignment as a "continuum" with "shared goals and expectations" across grades. The case studies of Fresno Unified School District (FUSD) and Long Beach Unified School District (LBUSD) are included in this report because of their noteworthy practices to support early childhood education and PreK-3 alignment. Four best practices implemented by districts contributed to their success: (1) continuous parent involvement; (2) joint professional development between Pre-K, Head Start, Child Care, TK, and K-3 teachers; (3) strong leadership commitment and training for principals; and (4) efforts to create a uniform and continuous district assessments and longitudinal data system. PreK-3 Alignment Interview Questions are appended. [For the summary of the report, "Opportunities to Align California's PreK-3 Education System," see ED574786.]