It is essential that all students who are interested in science, technology, engineering, and math (STEM) have the motivational supports to persist in their STEM career trajectories. "STEM role models," or successful individuals in STEM, can be powerful tools to motivate college students towards STEM careers, particularly if students feel similar to those role models. However, limited research has examined what dimensions of role model similarities are most optimally supportive of students' motivation and how to leverage similarity in STEM role model programming. This two-study dissertation investigated how perceptions of similarity to STEM role models motivates college students to pursue STEM careers and how educators might leverage students' perceptions of similarity to enhance the motivational power of STEM role models. Study 1 investigated the different ways in which college STEM students (N = 1,185) perceived similarity to STEM role models, how perceptions of similarity were related to students' motivation, and whether patterns varied for different gender and racial/ethnic groups in STEM. Results showed that perceptions of academic similarity (e.g., in terms of academic abilities, interests, or efforts) were most robustly associated with college students' STEM career motivation. This study also revealed that college students from marginalized gender and racial/ethnic groups may rely on different dimensions of academic similarity when being motivated by STEM role models. Study 2 investigated one technique through which educators might promote perceptions of similarity to a given STEM role model. This study experimentally tested a novel self-reflective intervention activity aimed to promote perceptions of similarity among college students enrolled in computer science courses (N = 406) and investigated whether effects of the activity varied for different groups of students. Despite overall null effects of the intervention activity, mixed effects of role model exposure revealed insights regarding the boundary conditions through which role model activities promote college students' motivation and career-related intentions in STEM. Both studies shed light how perceptions of similarity can make STEM role models powerful motivators to college students. Findings help to inform the design of effective role model programs that aim to motivate and broaden STEM participation among diverse groups of college 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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]