In recent years, several states have expanded a new publicly funded learning option: Transitional Kindergarten (TK). TK programs bridge prekindergarten and kindergarten in their eligibility, requirements, and design. We use Michigan’s TK program as a case study on the fit of this new entrant in the early learning landscape. Michigan’s program is well suited for this purpose because it contains most of the research-aligned program design elements of other large-scale TK programs across the country. Using statewide administrative data, we describe which districts offer TK when doing so is optional, the characteristics of children who enroll in TK at four and five years old, and substitution patterns between TK and alternative learning options. Broadly, we find TK in Michigan closes some socioeconomic gaps in early program enrollment while exacerbating others. Specifically, districts with larger proportions of White students and smaller proportions of economically disadvantaged students are more likely to offer TK than other districts. Within districts that do not offer TK in every school, TK is targeted to schools with more economically disadvantaged students. Among preschool-age children, those from non-economically disadvantaged families are more likely to enroll in TK than their peers; among kindergarten-age children, there is little difference in take-up by family income. Finally, we find evidence of substitution, with some children enrolling in TK instead of state-funded prekindergarten or instead of enrolling in kindergarten early, though with no evidence that public slots decline overall. Our findings have implications for addressing the fragmented early education landscape when expanding programs. (author abstract)