Science education in the United States has endured substantial reform due to national needs for a bolstered, more diverse science, technology, engineering, and mathematics workforce. Recent reform efforts call for students to be engaged in scientific inquiry through the practices of scientists and engineers. Opportunity gaps exist in science education and in the STEM pipeline for those who have traditionally been underrepresented, especially females and minorities. Research highlights the potential of science fair experiences to engage students in inquiry learning that could meet the needs of updated standards. Furthermore, students from diverse backgrounds might benefit from out-of-school time science learning opportunities such as science fair. Research results suggest students pursue STEM education and STEM careers due to development of STEM identity, and expectancy-value theory. Other research that investigated students' motivations to do science fair has failed to differentiate between compulsory and voluntary participation. Little is known about why students, choose to engage in science fair. This research used constructivist grounded theory to develop a theory of students' motivation to continually participate in scientific research for science fair. Data was collected and analyzed from intensive interviews of 23 students across eight school districts all within a Great Plains state. Participants engaged in multiple science fairs while in middle and/or high school and participated voluntarily for at least one iteration of science fair. Findings yielded a theoretical model that depicts the processes that students experienced as they engaged in science fair, such as the chance to pursue meaningful research, the challenges they faced, such as a lack of resources, and the support they received from sponsors. This study contributes to the literature on motivation to do science fair as findings indicate the development of science identity, students' needs to have autonomy in their research topic, and students' realization of scientific research as a tool to solve meaningful problems and science fair as the venue to be an expert. [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.]