This chapter explains maltreatment as any form of physical, sexual, or emotional abuse and/or neglect, is associated with many negative outcomes, especially poor mental health. By understanding the impact of maltreatment on the developing brain, social workers can identify some of the underlying mechanisms of the psychiatric problems common in this population, and provide more effective treatment. Externalizing disorders, such as attention-deficit hyperactivity disorder (ADHD), oppositional defiant disorder (ODD), and conduct disorder, are the most common diagnoses given to children in the child welfare system but internalizing disorders, such as anxiety, depression, and posttraumatic stress disorder (PTSD), are frequently diagnosed as well. Hypothalamic-pituitary-adrenal (HPA) axis is one of the major pathways through which the effects of stress can shape brain development. The chapter illustrates how social workers can incorporate findings from neurobiological research into their clinical practice with maltreated children through the use of a case study.