The purpose of this study was to accomplish three major objectives: to quantitatively understand the applicability of an existing list of characteristics of mental toughness to a population of ultramarathon runners, to establish a new definition of mental toughness specific to ultra runners, and to conduct interviews to understand experiences, examples, and development of mental toughness. A mixed-methods approach was utilized with online surveys and semi-structured interviews. Participants were ultramarathon runners who had completed at least one organized ultramarathon event of 50 miles or more, all of whom were at least 18 years of age. The online survey yielded 408 completed questionnaires and 476 definitions of mental toughness in ultramarathon running. The online survey consisted of an open-ended question on defining mental toughness, and a list of 30 characteristics of mental toughness from Jones and colleagues' (2007) mental toughness study on elite athletes. The survey utilized a 5-point Likert scale, and a Basic Demographic Questionnaire in addition to questions relevant to running experience. The existing characteristics represented limited applicability to the population of ultramarathon runners, with most prevalent attributes on drive towards success, goal attainment, learning from failure, and coping under pressure. In the 476 definitions of mental toughness, the most prominent themes were persistence, perseverance, utilization of psychological skills, and adapting to environmental and duration-related obstacles. Semi-structured interviews were conducted with 12 randomly selected ultramarathon runners. Interviews were designed to verify a raw definition of mental toughness in ultramarathon runners, identify traits of mentally tough ultra runners, understand experiences and examples of mental toughness, and understand how mental toughness could be developed. Grounded theory was utilized to isolate themes; many factors emerged and were categorized based on research questions. The definition of mental toughness was supported with the addition of an emotional and perceptual component. Traits of mentally tough runners supported the traits found in the definitions of mental toughness. Experiences and examples of mental toughness included situations of using psychological skills and witnessing a runner overcome adversity. Lastly, development of mental toughness was most highlighted in social aspects of the ultra culture and experiences both in running and in life. / Kinesiology