There is considerable concern that many adolescents do not attain the writing competence needed to be successful in school, their personal lives, or the workplace. Ensuring that students acquire this competence is a basic responsibility of schools. In order to meet this objective, teachers need access to effective practices for teaching writing. In this meta-analysis, we examined if teaching writing improved the writing and reading of students in Grades 6-12, and what specific writing treatments enhanced students' writing. Our review included writing treatments tested using an experimental or quasi-experimental design (with pretests) and published and unpublished studies, and computed effect sizes (ESs) for all writing and reading outcomes assessed. Across 406 independent comparisons, yielding 3,514 ESs involving 52,529 students, teaching writing had a positive and statistically detectable impact on students' writing (ES = 0.47) and reading (ES = 0.22). Moreover, a variety of different writing treatments improved students' performance on writing measures. Across all writing outcomes, statistically detectable effects (presented in parentheses) were obtained for comprehensive writing programs (0.47; which included the process approach to writing), strategy instruction (0.76), digital writing tools (0.31), transcription instruction (0.71), computer-assisted instruction (0.32), teaching critical/creative thinking skills for writing (0.27), emulating good models of writing (0.46), feedback (0.34), goal setting (0.44), prewriting activities (0.49), grammar instruction (0.77), sentence instruction (0.73), inquiry (0.92), observing writers/readers, peer assistance (0.41), summarization instruction (0.49), and text structure instruction (0.39). Implications for practice, research, and theory are discussed.