Peer review is seen as an important quality-assurance mechanism in both industrial development and the open-source software (OSS) community. The techniques for performing inspections have been well studied in industry; in OSS development, software peer reviews are not as well understood. To develop an empirical understanding of OSS peer review, we examine the review policies of 25 OSS projects and study the archival records of six large, mature, successful OSS projects. We extract a series of measures based on those used in traditional inspection experiments. We measure the frequency of review, the size of the contribution under review, the level of participation during review, the experience and expertise of the individuals involved in the review, the review interval, and the number of issues discussed during review. We create statistical models of the review efficiency, review interval, and effectiveness, the issues discussed during review, to determine which measures have the largest impact on review efficacy. We find that OSS peer reviews are conducted asynchronously by empowered experts who focus on changes that are in their area of expertise. Reviewers provide timely, regular feedback on small changes. The descriptive statistics clearly show that OSS review is drastically different from traditional inspection.