Vocabulary is essential for communicating, reading, thinking, and learning. In comparison to typical hearing peers, students who are deaf or hard of hearing demonstrate vocabulary knowledge that is quantitatively reduced. The authors review and summarize research studies published in peer-reviewed journals between 1967 and 2008 focusing on vocabulary and students who are deaf or hard of hearing. Forty-one studies are examined. A summary of each study is presented in a table, and potential educational implications are described. The authors note the paucity of research to guide instruction and provide suggestions for future research. (Contains 1 table.)