Word boxes (also known as sound boxes or Elkonin boxes) is a widely used intervention targeting basic literacy skills. The word/sound box intervention has been implemented across grades, settings, and student ability levels, but to date there has not been a quantitative analysis of the effects of the word/sound box intervention. The purpose of the current study was to evaluate the effectiveness of the word/sound box intervention through a systematic review and meta-analysis and determine if any moderator variables affected the effectiveness of the intervention. A systematic review and meta-analysis of single-case design studies on the word/sound box intervention was conducted. Results show that the word/sound box intervention was highly effective. Significant moderators included adherence to What Works Clearinghouse standards and grade level. Applications, limitations, and future directions for the word box intervention are discussed.