Although peer reviewing of writing is a way to create more writing opportunities in college and university settings, the validity and reliability of peer-generated grades are a major concern. This study investigated the validity and reliability of peer-generated writing grades of 708 students across sixteen different courses from four universities in a particular scaffolded reviewing context: Students were given guidance on peer assessment, used carefully constructed rubrics, and were provided clear incentives to take the assessment task seriously. Distinguishing between instructor and student perspectives of reliability and validity, the analyses suggest that the aggregate ratings of at least four peers on a piece of writing are both highly reliable and as valid as instructor ratings while (paradoxically) producing very low estimates of reliability and validity from the student perspective. The results suggest that instructor concerns about peer evaluation reliability and validity should not be a barrier to implementing peer evaluations, at least with appropriate scaffolds. Future research needs to investigate how to address student concerns about reliability and validity and to identify scaffolds that may ensure high levels of reliability and validity.