Science, technology, engineering, and mathematics (STEM) influence almost every aspect of our daily lives. However, despite the high demand for STEM occupational talent, the STEM pipeline continues leaking, with less than one-sixth of high school students pursuing STEM majors and only 50% of entering STEM majors matriculating into STEM fields. Science identity has been identified as the most powerful predictor of high school students pursuing an undergraduate STEM major. Yet, the construct remains largely ill-defined and unexplored. The purpose of this study was to develop the SciID Scale, a valid and reliable new instrument that measures a high school student's science identity. Subject experts and a small group of high school students provided content validation for the scale. Exploratory factor analysis was used which revealed an optimal two-factor solution, reflecting the traditional two-dimensions of identity theory: Exploration and Commitment. Structural equation modeling, regression analysis and contingency tables were used to confirm the convergent and divergent validity of the instrument with external variables. Lastly, a latent class analysis provided further validation of the scale as it yielded an optimal four-class solution that reflected traditional identity theory statuses of: Achieved, Foreclosed, Moratorium, and Diffused. These validation measures combined with the good reliability scores of each factor yielded the SciID Scale a valid and reliable instrument specifically designed for high school students. [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.]