Avoidant/Restrictive Food Intake Disorder (ARFID) is a new diagnosis, added to the fifth edition of the Diagnostic and Statistical Manual in 2013. ARFID is diagnosed in individuals whose limited food intake or narrow diet leads to weight loss, nutritional deficiency, dependence on nutritional supplements, or significant interference with daily functioning. ARFID is diagnosed when the eating restrictions are not caused by dissatisfaction with body shape or weight, but by 1) rejection of foods based on their sensory properties (picky eating), 2) limited appetite or apparent lack of interest in eating, or 3) fear of negative consequences, such as choking, vomiting, or gastrointestinal distress, from eating. To date there is little evidence that these behaviors are distinct from other forms of disordered eating, or that they lead to the nutritional and psychosocial symptoms of ARFID, particularly in adults. This dissertation provides initial evidence for the ARFID diagnosis, and identifies potential transdiagnostic mechanisms through which these eating behaviors might lead to ARFID symptoms. Chapters 1 and 2 present the descriptive psychopathology of ARFID symptoms related to adult picky eating. Chapter 1 demonstrates that adults with ARFID show similar levels of comorbidity and impairment to adults with symptoms of anorexia and bulimia, but that they can be differentiated from each other on measures of picky eating-specific eating behaviors. Chapter 2 shows that adult picky eaters report reduced dietary variety and intake of fruits and vegetables compared to non-picky eaters, and that picky eaters with ARFID symptoms report very low fruit and vegetable consumption, reduced protein consumption, and elevated snack and dessert intake. Chapter 3 describes the development and validation of a new self-report measure of the three ARFID eating behaviors. Chapter 4 is a replication of the studies presented in Chapters 1 and 2 in a sample of adults with ARFID symptoms due to each of the three eating behaviors. In addition, this study highlights cognitive-affective and associative learning processes through which each of the three eating behaviors might lead to significantly reduced food intake and the development of ARFID symptoms.