Although evidence suggests that Head Start has demonstrated some success in promoting certain child outcomes, such as vocabulary development and social skills, recent reports indicate that Head Start children are performing approximately a full standard deviation below national norms on standardized measures of early mathematics (U.S. DHHS, 2005; 2006a; 2006b; 2010). The purpose of this study was to evaluate the effectiveness of an early numeracy intervention designed specifically for use in Head Start classrooms. Ninety-six children from six Head Start classrooms (three intervention and three comparison) participated in the study. During a 13-week period, intervention teachers facilitated one whole- or small-group activity and one transition activity per day. In addition, intervention teachers provided mathematics activities for children to explore independently at a math center. Child outcomes were measured before and after the intervention using the Test of Early Mathematics Ability -- 3rd Edition and the EARLI Numeracy probes. Teacher reports and classroom observations were used to assess implementation fidelity, quality of mathematics instruction in the classrooms, and teacher acceptability of the intervention. Hierarchical regression analyses were run to determine if intervention condition was a significant predictor of the criterion variables when controlling for pretest scores and demographic characteristics. Overall, children who participated in the intervention made significant gains on the post-test measures. However, the gains were not significantly greater than those made by children in the comparison condition. Descriptive data indicated that intervention teachers spent more time facilitating mathematics activities and provided higher quality instruction than comparison teachers. Classroom observation and teacher report indicated the intervention teachers conducted the intervention as designed; however, the accuracy of the teacher report data was questionable due to several inconsistencies in reporting. Intervention teachers reported satisfaction with the intervention, but indicated ways in which the intervention could be improved. (author abstract)