Classroom formative assessment (CFA) is considered to be a fundamental part of effective teaching, as it is presumed to enhance student performance. However, there is only limited empirical evidence to support this notion. In this effect study, a quasi-experiment was conducted to compare 2 conditions. In the treatment condition, 17 teachers implemented a CFA model containing both daily and weekly goal-directed instruction, assessment, and immediate instructional feedback for students who needed additional support. In the control condition, 17 teachers implemented a modification to their usual practice. They assessed their students' mastery of learning goals on the basis of half-yearly mathematics tests, and prepared weekly pre-teaching sessions for groups of low-achieving students. The posttests showed no significant differences in student performance between the 2 conditions after controlling for student and teacher characteristics. The degree of implementation of the CFA model, however, appeared to be positively related to the 5th-grade students' performance.