Background: Engineering self-efficacy, or the belief in one's own capabilities to complete engineering tasks, has been shown to predict greater motivation, academic performance, and retention of engineering students. Investigating the types of experiences that influence engineering students' self-efficacy can reveal ways to support students in their undergraduate engineering programs. Purpose/Hypothesis(es): The purpose of this study was to qualitatively examine how undergraduate engineering students describe the sources of their engineering self-efficacy and whether patterns in students' responses differed by gender. Design/Method: Participants (N = 654) were undergraduate engineering students attending two public, land-grant universities in the U.S. Open-ended survey questions were used to identify the events, social experiences, and emotions that students described as relevant to their engineering self-efficacy. Chi-square analyses were used to investigate whether response patterns varied by gender. Results: Students described enactive performances as their most salient source of self-efficacy, but interesting insights also emerged about how engineering students draw from social and emotional experiences when developing their self-efficacy. Women more often referred to social sources of self-efficacy and reported fewer positive emotions than did men. Conclusion: Findings suggest ways that educators can provide more targeted opportunities for students to develop their self-efficacy in engineering.