Districts across the country are using Preschool to Third Grade (P-3) approaches to address long-standing barriers to achieving positive outcomes for young children, for example by helping increase children's readiness for elementary school. P-3 approaches are promising for realizing these results because they demand investments in the systems supporting young children from birth to age eight and call for high quality, developmentally appropriate, and academically rigorous care and education. P-3 approaches also present significant challenges for the individuals and organizations responsible for their implementation, particularly the teachers working directly with young children in classrooms. Existing research has shown both the promise and challenge of P-3 but little on how to support teacher success during implementation. Using Sociocultural Learning Theory, this study involved an in-depth investigation into the efforts of one district that aimed to provide significant support to teachers in making the shifts that P-3 approaches demand. This study used a qualitative case study design to explore how one P-12 school district supported teacher leaders' learning of P-3 through a targeted professional learning program. Findings show that coaches who were directly supporting teacher leaders did appropriate and use the kinds of teaching-and-learning moves that support teachers shifting their practice in service of a new approach to instruction. While district leaders supported early learning generally and invested in improving the quality of teaching, coaches working with teacher leaders did not yet appear to see their job as helping build or create P-3 practice. This study contributes to--and expands upon--existing empirical literature showing how Sociocultural Learning Theory informs understanding of professional learning in P-3 settings and support for coaches working directly with teachers. This study suggests districts and stakeholders who aim to create more coherence across the P-3 continuum need to demonstrate a clear, concrete vision for teachers' practice specific to alignment across grade levels and utilize joint professional learning spaces to overcome the barriers between early learning and early elementary grades. [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.]