To determine whether formative writing assessments that are directly tied to everyday classroom teaching and learning enhance students' writing performance, we conducted a meta-analysis of true and quasi-experiments conducted with students in grades 1 to 8. We found that feedback to students about writing from adults, peers, self, and computers statistically enhanced writing quality, yielding average weighted effect sizes of 0.87, 0.58, 0.62, and 0.38, respectively. We did not find, however, that teachers' monitoring of students' writing progress or implementation of the 6 + 1 Trait Writing model meaningfully enhanced students' writing. The findings from this meta-analysis provide support for the use of formative writing assessments that provide feedback directly to students as part of everyday teaching and learning. We argue that such assessments should be used more frequently by teachers, and that they should play a stronger role in the Next-Generation Assessment Systems being developed by Smarter Balanced and PARCC.