Aim/Purpose: This mixed-methods research study examined impostor phenomenon during postdoctoral training in science, technology, engineering and mathematics (STEM) through the following research question: “What are the manifestations of the impostor phenomenon experienced during postdoctoral training in STEM?” Background: The impostor phenomenon occurs when competent, high-achieving students and professionals believe that they are fraud and will be exposed eventually. It involves fear of failure, lack of authenticity, feeling fake or fraud-like, denial of one’s competence, and is linked to lower self-esteem, mental health consequences, and lack of belonging. Methodology: This study was conducted with US-based postdoctoral trainees (or postdocs) using mixed-methods approach. The study examined aspects of impostor phenomenon among 43 postdocs by converging survey data using Clance Phenomenon Scale (CIPS) and qualitative data from semi-structured Impostor interviews from the same participants. Both convenience and snowball sampling were used. Majority of the participants were White, female, and from science disciplines. Interview findings were organized into themes using constant comparative method and analytic induction. Contribution: Findings pointed to the need for better designing professional development programs for postdocs that would: 1) address fears and insecurities due to impostor-feelings, 2) normalize conversations around perceived failure, judgment, and one’s lack of belonging, and 3) provide support with networking, mentoring, academic communication, and mental health challenges. Findings: Survey results indicated moderate to intense impostor-feelings; interviews found six triggers of the impostor phenomenon during postdoctoral training: 1. not pursuing new things, 2. not making social connections, 3. impaired academic communication, 4. not applying, 5. procrastination and mental health, and 6. feeling undeserving and unqualified. Current findings were compared with prior findings of impostor-triggers among PhD students who also experienced the first three of these challenges during doctoral training: challenges to applying newly learnt knowledge in other domains, reaching out for help, and developing skills in academic communication verbally and through academic writing. Recommendations for Practitioners: The office of postdoctoral affairs could design professional development programs and individual development plans for those experiencing the impostor phenomenon, focusing on strengthening skills (e.g., academic writing) in particular. There was an environmental and systemic dimension to the imposter phenomenon, perhaps more prevalent among women in STEM. The academy could devise ways to better support scholars who experience this phenomenon. Recommendation for Researchers: Research characterizing the qualitative characteristics of the impostor phenomenon across the STEM pipeline (undergrads, PhD students, postdocs, and faculty) would help understand if the reasons and manifestations of this phenomenon vary among differing demographics of students and professionals. Impact on Society: Organizations could focus on the training, development, mental health, and stressors among postdocs in STEM, particularly by focusing on career transition points (e.g., PhD to postdoc transition, postdoc to faculty transition), especially for those at-risk of experiencing this phenomenon and therefore dropping out. Future Research: Future research could examine how to manage or overcome the impostor phenomenon for students and professionals, focus on disciplines outside STEM, and investigate how socialization opportunities may be compromised due to this phenomenon. Longitudinal studies might characterize the phenomenon better than those that focused on the impostor phenomenon at a single time-point.