It is important for students to develop spelling skills, and spelling skills have been found to benefit the development of reading and writing skills. Past reviews and meta-analyses supported the effectiveness of spelling instruction at improving a variety of academic outcomes including spelling, reading, and writing. This review and meta-analysis contributes to this research and extends the current research on the impact of implementation characteristics on effectiveness. This review and meta-analysis included 81 studies (43 group design and 38 single-case design [SCD]). The meta-analysis indicated an average effect of spelling instruction and intervention that was small in the group design research (g = 0.319) and moderate in the SCD research (Tau-U = 0.578). Implementation and methodological characteristics were described, and the nature of the comparison conditions and dependent variable were identified as potential moderators. A significant sample of research represented a range of participant samples, implementation characteristics, instructional practices, and methodological attributes. These results suggest that experimental spelling approaches are generally a modest improvement on existing approaches and that little research has investigated the effectiveness of some best practices, including practices that support individualization of spelling instruction. © 2024 Wiley Periodicals LLC.