This chapter is the second of two foundational chapters, useful for engaging with the remainder of the book. It distinguishes between adversity and trauma, emphasising trauma as a psychological and physiological response to a deprivation of safety. It provides an overview of trauma definitions, diagnostic criteria and prevalence, particularly within justice-involved populations. While the aim is not to create more clinicians, understanding some of the symptom characteristics can help us to understand their presentation and the elevated risk of criminal justice involvement. The chapter explores how trauma manifests—including PTSD symptoms, such as hyperarousal and avoidance, and how these responses can influence behaviour in criminal justice settings. It highlights the clustering of trauma among justice-involved individuals and critiques the lack of criminological focus on trauma's role in offending. It argues for a reconceptualisation of trauma, not merely as individual pathology, but as a social and systemic issue that shapes offending trajectories.