Despite the widespread popularity of the National Association for the Education of Young Children's guidelines on Developmentally Appropriate Practices (DAP), research on the effects of these practices is limited. In this article we review studies assessing the effects of DAP. We find no evidence of consistent effects of DAP for cognitive or academic outcomes. Further, the existing research has not generally been critically reviewed and is presented as showing positive results even when reported findings are mixed. We discuss methodological issues relevant to the study of DAP and review published empirical studies on the effects of DAP. Nearly all of the studies reviewed included a nested data design, in which effects of classroom practices were evaluated on multiple children within each classroom, but only one study used analyses appropriate to this data structure. We conducted Monte Carlo simulations for studies reporting positive or negative effects of DAP to evaluate the potential effects of clustering. Results demonstrated that only a few of the reported significant effects of DAP would have remained significant had the analyses been conducted appropriately. Further, the need to examine differential effects of DAP for different children and the effects of the broader school context is highlighted. © 2005 by The University of Chicago. All rights reserved.