"Spelling Mastery" is designed to explicitly teach spelling skills to students in grades 1 through 6. One of several Direct Instruction curricula from McGraw-Hill that precisely specify how to teach incremental content, "Spelling Mastery" includes phonemic, morphemic, and whole-word strategies. The What Works Clearinghouse (WWC) identified two studies of "Spelling Mastery" that both fall within the scope of the Students with Learning Disabilities topic area and meet WWC evidence standards. These two studies meet standards without reservations. Together, these studies included 70 students with learning disabilities in grades 2 through 4 in three elementary schools or receiving instruction at a summer program. The WWC considers the extent of evidence for "Spelling Mastery" on the writing achievement of students with learning disabilities to be small for one outcome domain--writing. There were no studies that meet standards in the eight other domains, so they do not report on the effectiveness of "Spelling Mastery" for those domains in this intervention report. Appended are: (1) Research details for Darch & Simpson (1990); (2) Research details for Darch et al. (2006); (3) Outcome measures for the writing domain; (4) Findings included in the rating for the writing domain; and (5) Description of subtest findings for the writing domain. An explanation of the WWC rating criteria and a glossary of terms are also included. (Contains 5 endnotes and 5 tables.) [The two studies examined in this intervention report are: (1) Darch, C., & Simpson, R. G. (1990). "Effectiveness of visual imagery versus rule-based strategies in teaching spelling to learning disabled students." "Research in Rural Education," 7(1), 61-70; and (2) Darch, C., Eaves, R. C., Crowe, D. A., Simmons, K., and Conniff, A. (2006). "Teaching spelling to students with learning disabilities: A comparison of rule-based strategies versus traditional instruction." "Journal of Direct Instruction," 6(1), 1-16.]