This chapter introduces trauma as a foundational lens for understanding and supporting people who are homeless. It outlines trauma’s three core components—event, experience, and effect—and critiques dominant diagnostic frameworks (DSM-5 and ICD-11) for their limitations, particularly in excluding neglect and failing to account for developmental context and somatic impacts. The prevalence of trauma among people who are homeless is extraordinarily high, with studies indicating a disproportionate burden of adverse childhood experiences (ACEs), often involving abuse, neglect, and household dysfunction. Trauma serves both as a risk factor for and a consequence of homelessness, with ongoing exposure to violence, exploitation, and systemic neglect. The chapter explores the neurobiological and psychological imprint of trauma, including reenactments, attachment disruptions, and narrowed windows of emotional tolerance. Diagnostic complexities are discussed, including the distinctions between PTSD, complex PTSD (cPTSD), and borderline personality disorder. The chapter further addresses intergenerational and institutional trauma, highlighting the lasting effects of colonialism and systemic disempowerment, particularly for Indigenous populations. A trauma-informed approach is advocated to reframe behaviors as adaptive responses to trauma rather than signs of pathology, emphasizing the importance of trust, safety, and relational care in health care and social systems.