Suggestions from the self-determination theory and extant literature suggest the fulfillment of basic psychological needs (i.e., autonomy, competence, and relatedness) is necessary for well-being and to prevent ill-being. Autonomy is the perception that an individual is the source of their own behavior and can act in ways congruent with his or her sense of self. Competence is the sense of confidence one has in his or her abilities to be effective in their interactions within the environment. Relatedness is the sense of connection to important others, that one cares for and is cared for by others. Emerging adulthood is a unique developmental stage marked by instability, in which many common psychological disorders (i.e., ill-being) reach their peak. This dissertation explored the interconnections between basic psychological needs and well- and ill-being during emerging adulthood, using two distinct samples. The first study used a latent profile approach to identify subgroups of emerging adults based on their levels of basic psychological need satisfaction and frustration; the subgroups, known as profiles, were then compared with regard to their reports of well-being (i.e., life satisfaction and vocational identity) and ill-being (i.e., anxiety and depression). Using latent profile analysis, a two-profile solution was found using an exploratory sample of college students (N = 177). The first profile had significantly lower levels of basic psychological need satisfaction and higher levels of basic psychological need frustration than the second profile, both seem to generally reflect the theoretical notion that needs exist on a continuum from satisfied to frustrated. The two-profile structure was verified using a confirmatory sample of college students (N = 375). Significant group differences were then found using a multivariate analysis of variance (MANOVA), in that members of the profile with lower levels of basic psychological need satisfaction and higher basic psychological need frustration had significantly lower levels of life satisfaction and vocational identity and higher levels of anxiety and depression. Implications for theorists, clinicians, and college administrators and educators are discussed. The second study examined the role of an autonomy-thwartive environment, specifically hostile deployment experiences, and its' influence on soldiers' competence, conceptualized as performance, and mental health, as well as how these relationships differ based on relational environments. A sample of combat-exposed soldiers between the ages of 18 and 25 from the restricted military personnel Army Study to Assess Risk and Resilience among Servicemembers (STARRS) dataset (N = 5,284) was used. Findings suggested that higher levels of hostile deployment experiences were found to be related to a greater likelihood of anxiety, depression, and posttraumatic stress disorder (PTSD). Higher levels of hostile deployment experiences were also linked to poorer military performance. Poorer military performance was, in turn, related to a greater likelihood of anxiety, depression, and PTSD. Using moderated mediation structural equation modeling, buffering effects of the detrimental impact of hostile deployment experiences were found for both unit cohesion and interpersonal relational experiences. Implications for those working with military service personnel, specifically marriage and family therapists, are focused on ways to bolster unit cohesion and healthy interpersonal relationships as a protective factor for young service members.