Much of the currently available evidence on the causal effects of public prekindergarten programs on school readiness outcomes comes from studies that use a regression-discontinuity design (RDD) with the age cutoff to enter a program in a given year as the basis for assignment to treatment and control conditions. Because the RDD has high internal validity when its key assumptions are met, these studies appear to provide strong evidence for the effectiveness of these programs. However, there are overlooked methodological problems in the way this design has typically been applied that have the potential to produce biased effect estimates. We describe these problems, argue that they deserve more attention from researchers using this design than they have received, and offer suggestions for improving future studies.