Theories of writing development posit several component skills as necessary to the writing process. This meta-analysis synthesizes the literature on the correlation between these proposed component skills and writing outcomes. Specifically, this study examines the bivariate relationship of handwriting fluency, spelling, reading, and oral language with students' quality of writing and writing production. Additionally, the extent to which such relationships are moderated by student grade level and type of learner is also investigated. The findings document that each of the component skills demonstrates a weak to moderate positive relationship to outcomes assessing writing quality (rs = 0.33-0.49) and the amount students write (rs = 0.20-0.48). Moderator analyses were generally not significant with the exception that the relationship between reading and writing production was significantly higher for students in the primary grades. The implications of these findings to current theories and future research are discussed. [This paper was published in "Review of Educational Research" v86 p570-601 2016 (EJ1100191).]