The purpose of this research was to explore experiences of nontraditional doctoral students with impostor phenomenon (IP) at an R1: Doctoral University in the U.S. The research questions guiding the study were: (1) what are the experiences of nontraditional doctoral students with impostor phenomenon at an R1: Doctoral University in the U.S.? (2) how has impostor phenomenon affected nontraditional doctoral students - physically, emotionally, mentally, spiritually, academically, and professionally? and (3) what are the coping strategies that nontraditional doctoral students used to address their impostor phenomenon feelings and thoughts in the doctoral program? The researcher collected data via in-depth interviews with 20 nontraditional doctoral students at an R1: Doctoral University in the U.S. who were 30 years old (or older), had some work experiences prior to the doctoral program, had been in the doctoral program (not in the College of Education) for at least two semesters, and completed Clance's Impostor Phenomenon Scale (CIPS) to indicate the levels of their impostor characteristics. The researcher also conducted member-checking interviews with all the 20 participants in this study to verify the accuracy of the data interpretation. The findings indicate that all participants in this study experienced impostor phenomenon at different levels -ranging from frequently have intense IP experiences to frequently have impostor feelings, to having moderate IP experiences, and to having few impostor characteristics. Participants expressed internal, personal, and academic challenges that are associated with their impostor feelings and thoughts in the doctoral studies. Internal challenges refer to feelings of self-inadequacy, low self-esteem, self-judgement, and low self-compassion. Personal challenges include opportunity cost, relocation, and threats to identity. Academic challenges were described as a lack of academic support by advisor / mentor / PI, a lack of knowledge and/or experiences in the field of study, and challenges with academic reading and writing. The research findings also reveal physical, emotional, mental, spiritual, academic, and professional impacts of impostor phenomenon on nontraditional doctoral students at an R1: Doctoral University. Participants reported using self-help and external help as coping mechanism to address their impostor feelings and thoughts as well as impacts of those impostor phenomenon experiences in their doctoral studies. Based on the findings, the implications of this research include acknowledging the various aspects and tensions of impostor phenomenon that may impact nontraditional doctoral students at personal, academic, and/or professional levels. This research can be used as a reminder and/or guidelines to understand how the nature of academia may activate impostor phenomenon in (nontraditional doctoral) students. It is thus recommended that faculty, staff, educational leaders and/or those who are working closely with (nontraditional doctoral) students are open and/or willing to (re)consider practicing pragmatic support systems to help nontraditional doctoral students to cope with impostor phenomenon experiences. Future research studies should use more than one research tool or method to understand the impostor phenomenon experiences that nontraditional doctoral students may experience in different learning settings (location) and/or study how sub-populations of nontraditional doctoral students may perceive themselves as impostors. Future research should also investigate (the nuance of) nontraditional doctoral students' experiences with impostor phenomenon using different adult learning theories so that they may be able to suggest different ways to look at impostor phenomenon experiences and/or introduce how (different) adult learning theories can be used to explore, explain, and/or discuss impostor phenomenon experiences in doctoral studies. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]