This chapter defines dissociation and trauma-related disorders (TRD), discusses the etiology of TRD, and describes the prevalence and complex symptom presentations of individuals who have experienced complex trauma. Despite being at least as prevalent as many other types of mental illness, complex TRDs are underrecognized. Consequently, most of these individuals do not receive treatment specifically targeting the dissociative symptoms that maintain them. Due to a variety of factors, including lack of training for professionals, misunderstandings about dissociation, and a shortage of trauma-trained therapists, individuals with TRD are “hidden in plain sight” despite typically seeking mental health treatment for years. Furthermore, the shortage of clinicians trained in assessing and treating TRD creates a barrier for obtaining treatment that is of meaningful help in stabilizing trauma-related reactions including dissociation. The costs of being underrecognized and undertreated are enormous for dissociative individuals, the health care system, and society.