In response to concerns about poor engagement and achievement in reading, some high schools have implemented independent reading programs to give students time to read self-selected texts during school. Some schools are also adopting the Lexile framework as a tool for matching readers with challenging texts in order to improve reading motivation and achievement. While a variety of research has investigated the efficacy of independent reading programs like sustained silent reading (SSR), many researchers contend that there is still insufficient evidence to support the efficacy of such programs. In addition, there has been no peer-reviewed research on the effectiveness of using of Lexiles to match readers with challenging texts. This dissertation presents a study that explores how the challenge level of materials read relates to students' intrinsic motivation to read and reading to their reading achievement during independent reading in 9th and 10th grades. Control and treatment groups were established at a high school where sustained silent reading was already in place. The control group consisted of students who were free to choose their own reading materials during SSR. The treatment groups included students who were expected to read within their Lexile ranges. Results revealed that teachers were reluctant to require students to use Lexiles to choose texts. Despite this, 24% of students in the sample reported that Lexiles were helpful in finding texts for SSR. In addition, males were more likely to report that Lexiles were helpful than were females. Multiple least squares regression analyses found that students who tended to read texts that were more challenging and that students who perceived Lexiles to be more helpful tended to show greater increases in intrinsic motivation to read and reading achievement. Nonetheless, a variety of factors limit the generalizability of the results. The data support further research into challenge as an important variable to consider in future studies of independent reading. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]