This paper reviews research on outcomes of writing programs for students in grades 2 to 12. Studies had to meet rigorous standards of research including use of randomized or well-matched control groups, measures independent of the program developers, researchers, and teachers, and adequate sample size and duration. Fourteen studies of 12 programs met the standards. Twelve (86%) were randomized, two matched. Programs were divided into three categories. Student achievement effects on writing were positive on average in all categories (Effect Size=+0.18), with similar outcomes for writing programs focused on the writing process (ES=+0.17), those using cooperative learning (ES=+0.16), and those focusing on interactions between reading and writing (ES=+0.19).