We present the theory and implementation of a review strategy based on testing rather than lecturing. We also show the results of a beginning-of-course review using the format of a two-stage examination, in which students complete a set of questions individually, then again as a group. This format offers several benefits compared with the typical lecture review: (a) students engage with the review topics much more deeply and more accurately gauge their own preparation; (b) students receive immediate, corrective feedback from their peers and clarify their understanding through discussion during the group stage; and (c) the instructor receives detailed information on students' background understanding that can be used to tailor instruction. These proposed benefits are supported by the improved performance of groups during the second stage and by student opinions collected by survey several days after the review activity. The two-stage review therefore serves to both diagnose and remediate deficiencies in background understanding, leaving students and instructors better prepared for the course.