During this decade there have been numerous efforts to identify instructional practices that improve students' writing. These include "Reading Next" (Biancarosa and Snow, 2004), which provided a set of instructional recommendations for improving writing, and "Writing Next" (Graham and Perin, 2007) and "Writing to Read" (Graham and Hebert, 2010), which were systematic reviews of high-quality research that identified effective writing practices for improving both writing and reading, respectively. Despite these efforts and efforts by others (e.g., Bangert-Drowns, Hurley, and Wilkinson, 2004; Rogers and Graham, 2008), educators and policymakers need additional evidence-based practices for improving the writing of students in American schools. One tool with potential for improving students' ability to effectively convey thoughts and ideas through text is classroom-based writing assessment. Such formative assessments allow teachers to gauge the effectiveness of their instructional practices, modify instruction as needed, and provide students with feedback on writing strengths and areas in need of further development. These assessments can be administered in a variety of ways in the classroom, including teachers assessing students' writing, students assessing their own writing, and peers assessing others' writing. This report provides evidence to answer the following two questions: (1) Does formative writing assessment enhance students' writing?; and (2) How can teachers improve formative writing assessment in the classroom? This is the first report to examine the effectiveness of formative writing assessment (question 1) using the powerful statistical method of meta-analysis. This technique allows researchers to determine the "consistency" and "strength" of the effects of an instructional practice, and to highlight practices holding the most promise. This report also identifies best practices in writing assessment that need to be implemented in order to maximize the accuracy and trustworthiness of formative writing assessment (question 2). Appended are: (1) Methodology; and (2) Studies and Data Examined in the Report. (Contains 15 tables.)