Students are not achieving at appropriate levels in mathematics. Among the explanations for this deficiency is that current curriculum and teaching strategies do not engage students at the appropriate level of understanding. The van Hiele model is a Geometry-specific learning model that outlines five levels of understanding through which students must progress in order to effectively learn Geometry. This pretest-posttest control group study explored how van Hiele-based Geometry instruction effected student understanding gained and student achievement gained. Specifically, research questions investigated how van Hiele-based Geometry instruction effected student van Hiele levels and student Geometry achievement levels. The Van Hiele Geometry Test and the Entering Geometry Student Test were each used in a pretest-posttest format to measure entering and exiting student van Hiele levels and student Geometry achievement levels, respectively. Gain values, the difference in exiting and entering values from each assessment, were analyzed using independent samples t-tests. Data were collected during a 20 day intensive Geometry course taught at Polaris Evening School, Woodstock, Georgia. The population was divided into two instructional groups, van Hiele and traditional. Results of this study indicated van Hiele-based instruction produced no significant differences in student understanding gained or student achievement gained. However, social change was affected in the sense that this study showed van Hiele-based instruction as a viable alternative to traditional instructional methods. That is, this study affected the education community by broadening the arsenal of instructional strategies for educators.