Research Findings: The early childhood years are critical in developing early mathematics skills, but the opportunities one has to learn mathematics tend to be limited, preventing the development of significant mathematics learning. By conducting a meta-analysis of 29 experimental and quasi-experimental studies that have been published since 2000, this study extends beyond prior evaluations of early mathematics programs for prekindergarten and kindergarten environments by quantifying program effectiveness in terms of effect sizes and examining 6 aspects of these programs. We found an overall moderate to large effect size. There was a tendency for programs to produce larger effects when they (a) targeted a single content strand, (b) presented content 120 to 150 min per week, (c) designed programs for the prekindergarten environment, (d) presented content individually to children, and (e) used researcher-made mathematics assessments. Practice or Policy: Early mathematics programs can be designed to be both developmentally appropriate and highly effective. The goal of this meta-analysis was to reveal these programs to early development and education professionals so that they understand some of the factors that might explain why some produced stronger effects than others.