Developmental science and school research identify children's transition to kindergarten as a sensitive period with significant implications for formal school success. In this chapter, we present evidence that a successful transition to kindergarten requires more than ensuring that children have requisite competencies. Instead, we present an ecological model that conceptualizes smooth transitions from pre-kindergarten to kindergarten as a function of linkages between systems, such as connections between schools and families and between pre-kindergarten and kindergarten teachers and classrooms, especially those made prior to kindergarten entry. This chapter provides an overview of research on and best practices for effective pre-kindergarten to kindergarten transitions that support children's development and school readiness. Evidence for the ecological framework supporting this critical transition is provided, including how transition and alignment practices are associated with optimal outcomes for children. Promising practices from the field that promote alignment between pre-kindergarten and kindergarten experiences are also addressed. Additionally, we present several case studies detailing the ways in which different localities have used empirically supported transition practices to support children's early school success. Finally, we conclude with thoughts regarding future directions for transition and alignment work in early childhood.