We investigated the effect of writing interventions on written composition for students in primary grades (K-G3) with a focus on whether effects vary as a function of different dimensions of composition outcomes (i.e., quality, productivity, fluency, and other), instructional focus (e.g., transcription, self-regulation strategies such as Self-Regulated Strategy Development [SRSD]), and student characteristics (i.e., initially weak writing skills). A total of 24 studies (number of effect sizes, k = 166; N = 5589 participants) met inclusion criteria. The overall mean effect size was moderate and positive (ES = 0.31) with some variation across the dimensions of composition: 0.32 in writing quality, 0.31 in writing productivity, 0.15 in writing fluency, and 0.34 in writing: other. SRSD had large and consistent effect sizes across the outcomes (0.59-1.04) whereas transcription instruction did not yield statistically significant effects on any dimensions of composition due to large variation of effects across studies. Variation in instructional dosage (total length of instruction) did not explain variation in the effect sizes. Lastly, the average effect on writing quality was larger for writers with weaker writing skills compared to those with typical skills.