This chapter is foundational to the remainder of the book and explores the concept of Adverse Childhood Experiences (ACEs). It proposed that ACEs are a critical deprivation of safety that contribute to long-term negative outcomes, including, serious and violent offending, and involvement in the criminal justice system. The chapter reviews the historical evolution of ACE research, highlights the prevalence and impact of ACEs in conflict-affected regions such as Northern Ireland, and critiques the oversimplification of complex life events often the subject of analysis in ACE frameworks. Key limitations—such as retrospective bias, cultural insensitivity and failure to distinguish causation from correlation—are discussed. The chapter concludes by arguing that while ACEs are vital for identifying harm during childhood, they are insufficient alone to explain why some individuals develop offending patterns while others do not.