The recent Next Generation Science Standards recommend students have increased opportunities engaging in modeling work scientists do. Science classroom tasks include simple inquiry compared to more complex authentic inquiry that closely resembles authentic science. Science fair participation presents students with the opportunity to engage in authentic inquiry through novel experimentation. Despite the popularity of science fairs, educational research has neglected to focus on it as a valuable enterprise for understanding student experimental reasoning during authentic inquiry. This mixed methods research study explored science fair participation as a viable option for understanding how students demonstrate knowledge and application of experimental reasoning elements while engaging in authentic inquiry. Toulmin's Argument Pattern was utilized as a framework to examine how experts and novices understand and apply experimental reasoning elements associated with the scientific method. Science fair judges and students responded to a Likert type survey item regarding student understanding of design and data reasoning specific elements. One-way ANOVA analysis showed significant differences between novices reporting of their understanding of design and data reasoning elements and experts reporting of novices understanding of the same reasoning elements. Further analysis showed significant differences between understanding of design reasoning items compared to data reasoning items for both novices and judges. To explore the application of experimental reasoning, science fair judging criteria, results, and comments were used in conjunction with experts' importance rankings of design and data reasoning elements. Utilizing a descriptive case study approach, data was coded and analyzed for how experts and novices applied experimental reasoning elements. Emerging themes centered on both novices and experts applying experimental reasoning unevenly. Novices applied design reasoning elements more frequently compared to data reasoning elements. Experts also applied design reasoning elements and general presentation items more frequently, as well as prioritizing them according to ranking analysis. If students are to have increased opportunities to engage in authentic inquiry, science fairs have the potential to be a foundational component of science curriculum. Knowing more about students' understanding and application of experimental reasoning has direct implications for educational researchers, science educators, and science fair administrators. [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.]