This study investigated the effectiveness of a unit plan designed to enhance students' three-dimensional geometry and spatial ability skills. Research participants consisted of 50 students, 19 males and 31 females, and two teachers from a public high school in a suburban middle-class community. The students in this study were 9th and 10th graders in one Geometry College Preparatory course and two Geometry Honors courses. Students were given The Purdue Visualization of Rotations Test, a reasonable indicator of spatial ability, as a pre-test and a post-test, to discover whether or not students demonstrated a significant increase in spatial ability. The researcher and two teachers in the study created a geometry unit plan focused on moving from two-dimensional figures to three-dimensional visualization. Quantitative results indicated that the unit plan created for this study was effective. Overall, student scores increased from pre-test to post-test one. Evidence from the study suggests that well-designed components can be helpful in improving spatial skills. Evidence from the qualitative results suggested that more time should be devoted to solving spatial ability problems. Unit plans that focus on spatial ability should provide students with problems that require them to use various spatial ability concepts. Responses to interview questions suggest that most students used similar approaches when solving spatial ability problems. Mentally manipulating figures to find solutions was a commonly used strategy, as well as paper and pencil drawings. Based on quantitative results, retention of knowledge was not significant, perhaps due to the study occurring at the end of the school year. [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/en-US/products/dissertations/individuals.shtml.]