United States high schools are increasingly using online learning to complement traditional classroom learning. Previous researchers of post secondary online learning have shown no significant differences between traditional and online learning. However, there has been little research at the secondary level about the effectiveness of online learning. The purpose of this quantitative quasi-experimental study was to determine the effectiveness of online learning compared to traditional learning overall and by gender using pre and posttest scores from online and traditional learning environments. This research is based on the learning theory of constructivism because online learning is seen as a good example of constructivist learning. The sample, 111 high school seniors, was randomly assigned to either an online learning class or traditional learning classroom. Scores from the pre and posttests were analyzed using ANCOVA to determine statistical differences between the 2 groups. The results of the analysis indicated no significant difference between the online and traditional learning groups overall or when examined by gender. Both groups and both genders demonstrated equal advancements in academic progress, illustrating that the online and traditional classroom can provide equivalent learning opportunities. These results are consistent with the findings of other online learning studies. The results from this study should be of interest to educators and school districts considering online learning as an alternative education model in high schools. Positive social implications are a better understanding of online learning as an effective learning environment for secondary students, thus offering an alternative particularly for those students who struggle to find success in traditional classroom settings. [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.]