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. (author abstract)