The purpose of this study is to determine if Project-Based Learning (PBL) retrieval activities can impact knowledge gains and retention in high school mathematics students, and to look for a possible relationship between student achievement level, retrieval activity type, and mathematics knowledge retention. For two mathematical topics, ten high school seniors (six average-achieving and four high-achieving) received instruction, completed a post-instruction test, participated in either a PBL or a Traditional retrieval activity, and completed a post-activity test the day after the activity to measure knowledge gains and a deferred post-test six weeks after the activity to measure knowledge retention. There was no statistically significant difference in either knowledge gains or knowledge retention based on type of retrieval activity. The knowledge gains effect size between the two groups was negligible (Hedges' g = 0.017). The knowledge retention effect size was small (Hedges' g = 0.203) with a preference for the Traditional activity. Within both achievement levels, large effect sizes (Hedges' g > 0.8) were found in knowledge retention, with the high-achieving group showing higher knowledge retention after the PBL activity and the average-achieving group showing higher knowledge retention after the Traditional retrieval activity. These results indicate that PBL activities can be as effective as Traditional activities in the high school mathematics classroom and may be more beneficial to some students. Future research to determine attributes that may contribute to student's response to different types of retrieval activities is suggested. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]