California’s Transitional Kindergarten (TK) program provides an additional year of schooling within the K–12 system that aims to prepare children for kindergarten. Launched a decade ago with limited eligibility, the program will be expanded to all four-year-olds by 2025–26. Little is known about TK’s longer-term impact—especially among multilingual students and students with disabilities, who might benefit from early identification. Taking stock of TK’s impact so far can help the state expand it successfully.