Having students express their understanding of difficult, new material in their own words is an effective method to deepen their comprehension and learning. Summary Street® is a computer tutor that offers a supportive context for students to practice this activity by means of summary writing, guiding them through successive cycles of revising with feedback on the content of their writing. Automatic evaluation of the content of student summaries is enabled by Latent Semantic Analysis (LSA). This article describes an experimental study of the comprehension and writing tutor, in which 8th-grade students practiced summary writing over a 4-week period, either with or without the guidance of the tutor. Students using Summary Street® scored significantly higher on an independent comprehension test than the control group for test items that tapped gist level comprehension. Their summaries were also judged to be significantly superior in blind scoring on several measures of writing quality. Students of low-to-moderate achievement levels benefitted most from the tool.