In an effort to identify effective instructional practices for teaching writing to elementary grade students, we conducted a meta-analysis of the writing intervention literature, focusing our efforts on true and quasi-experiments. We located 115 documents that included the statistics for computing an effect size (ES). We calculated an average weighted ES for 13 writing interventions. To be included in the analysis, a writing intervention had to be tested in 4 studies. Six writing interventions involved explicitly teaching writing processes, skills, or knowledge. All but 1 of these interventions (grammar instruction) produced a statistically significant effect: strategy instruction (ES = 1.02), adding self-regulation to strategy instruction (ES = 0.50), text structure instruction (ES = 0.59), creativity/imagery instruction (ES = 0.70), and teaching transcription skills (ES = 0.55). Four writing interventions involved procedures for scaffolding or supporting students' writing. Each of these interventions produced statistically significant effects: prewriting activities (ES = 0.54), peer assistance when writing (ES = 0.89), product goals (ES = 0.76), and assessing writing (0.42). We also found that word processing (ES = 0.47), extra writing (ES = 0.30), and comprehensive writing programs (ES = 0.42) resulted in a statistically significant improvement in the quality of students' writing. Moderator analyses revealed that the self-regulated strategy development model (ES = 1.17) and process approach to writing instruction (ES = 0.40) improved how well students wrote. (Contains 2 tables.)