California’s Transitional Kindergarten (TK) program provides an additional year of schooling aimed at preparing children for kindergarten within the K–12 system. TK was launched a decade ago with limited eligibility, but it will be open to all four-year-olds by 2025–26. While early research has shown that TK improved kindergarten readiness, little is known about its longer-term impact—especially among multilingual students and students with disabilities, who might benefit from early identification of special needs. To learn more, we followed initial TK cohorts through elementary school in five large school districts. (author abstract)