Background: Many undergraduate research (UR) students are mentored by graduate students or postdoctoral researchers, yet few studies have examined the mentoring knowledge, skills, and attributes (KSAs) of mentors in UR settings or assessed their beliefs about their mentoring KSAs. Purpose: The purpose of this study was to identify important mentoring KSAs for graduate students and postdoctoral researchers who serve as mentors in engineering and science UR settings and to use these KSAs to develop a survey that mentors can use to assess their beliefs about their mentoring KSAs. Design/Method: In Phase I of the study, semistructured interviews were conducted with one postdoctoral researcher and 16 graduate students who were recognized by their students as outstanding mentors. In Phase II, survey items were created from the results of Phase I for assessing the beliefs of graduate students and postdoctoral researchers about their mentoring KSAs. Results: The study identified important mentoring KSAs and produced a 30-item survey that assesses the beliefs of mentors about their mentoring KSAs; the items are grouped into four factors: building a positive working relationship, recognizing individual student needs and personalizing the mentoring approach, monitoring students' daily tasks, and building a personal relationship with students. Conclusions: The study findings can help graduate students and postdoctoral researchers to become better mentors in UR settings by identifying important mentoring KSAs and by enabling them to assess their beliefs about their mentoring KSAs.