Project-based learning (PBL) offers an ideal approach for cultivating a robust STEM workforce, empowering students to actively construct their knowledge. This process has the potential to enhance science identity and self-efficacy. Furthermore, because PBL is inherently collaborative, it fosters the development of 21st-century skills, many of which align with social and emotional learning (SEL). While past studies have explored the relationship between PBL, SEL, science identity, and self-efficacy, there is a research gap in examining all three variables collectively within a high school context. Thus, this mixed-methods action research study aimed to determine if the implementation of a PBL unit, with specific inclusion of SEL skills, had an effect on high school AP Biology student SEL skill development, science identity, and science self-efficacy. Quantitative pre-post unit data on student SEL skills, science identity and science self-efficacy, along with qualitative data from focus group interviews, student reflection journals, and teacher notes and observations indicate the PBL unit resulted in significant and positive changes in each area of interest. This research suggests implementing PBL with purposeful inclusion of SEL skills can significantly increase student SEL skills as well as their feelings of confidence in science and science identity. [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.]