Enhancing writing pedagogy in university-based courses for students of English as a foreign language has been a long-term goal of educators and researchers. Most recent work, however, has relied on qualitative methodologies that limit our capacity to generalize findings. In this quantitative study, the authors adopted a quasi-experimental control group design to investigate effects of peer feedback and author response on university students' writing performance in a context of English as a foreign language. A total of 198 sophomores majoring in English took part in a 15-week research study. The authors conducted a repeated-measures analysis of covariance to examine the effects of a traditional teacher feedback instructional group compared with an experimental peer feedback group. Results showed that the peer feedback group showed a significantly greater writing performance gain than the traditional teacher feedback group. Furthermore, follow-up analyses focusing on the experimental group that relied on a series of multiple regressions showed that both the quality of students' feedback and the quality of their author responses were significant and stable predictors of students' writing performance. Results also showed that the quality of students' author responses had a slightly larger impact on writing performance than the quality of students' feedback. This study provides clear evidence of the power of formative peer feedback as an approach to support learning that both empowers students and addresses widely cited problems that arise when the instructor is the only source of feedback in a writing course.