Although students from underrepresented groups (URGs) matriculate into higher education at continually increasing numbers, the undergraduate science, technology, engineering, and mathematics (STEM) degree completion rates for students from URGs remain low. In the United States, institutions of higher education (IHEs) continue to face the challenge of retaining and graduating students from URGs enrolled in STEM programs. Some have offered summer bridge programs (SBPs) to improve retention. This dissertation study addressed one main research question-- How does a STEM SBP impact students from URGs during their transition into the first semester of college?--with four subquestions. The setting was a large public research institution in the mid-Atlantic region of the United States. A mixed-methods approach was used. Although 31 students were recruited, complete qualitative data were available only for 11 students, who comprised the final participant group. Qualitative data were collected through interviews, focus groups, observations, participant texts, and field notes. Quantitative data were collected using pre- and post-tests for an SBP survey, General Self-Efficacy Survey, and STEM Semantics Survey. Quantitative data results revealed a statistically significant increase after the SBP in math and engineering interest from the STEM Semantics Survey. The qualitative data demonstrated that most study participants noted an increase in self-efficacy and confidence after attending the STEM SBP. In addition, study participants found that the social connections, academic preparation, and mentorships with faculty and staff at the STEM SBP further increased their self-efficacy and prepared them for their first semester. The STEM SBP improved the students' sense of belonging by introducing them to STEM content and real-world activities as well as to classmates and mentors. Finally, study participants had other experiences to contend with that impacted their first semester, such as family pressures, the lack of diverse STEM role models, and discrimination based on gender, race, and sexual orientation. Based on the findings, a checklist is offered for IHEs seeking to implement SBPs based on key findings identified in this study. This study adds to the current academic discussion on STEM retention and raises awareness on how IHEs can assist in feeding the STEM pipeline with a heterogeneous workforce. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]