Students' questions play an important role in meaningful learning and scientific inquiry. They are a potential resource for both teaching and learning science. Despite the capacity of students' questions for enhancing learning, much of this potential still remains untapped. The purpose of this paper, therefore, is to examine and review the existing research on students' questions and to explore ways of advancing future work into this area. The paper begins by highlighting the importance and role of students' questions from the perspectives of both the learner and the teacher. It then reviews the empirical research on students' questions, with a focus on four areas: (1) the nature and types of these questions; (2) the effects of teaching students questioning skills; (3) the relationship between students' questions and selected variables; and (4) teachers' responses to, and students' perceptions of, students' questions. Following this, some issues and implications of students' questions for classroom instruction are discussed. The paper concludes by suggesting several areas for future research that have significant value for student learning. (Contains 4 tables.)