This chapter provides an overview of the neurophysiology underlying the innate human response of anxiety. Knowledge regarding the brain’s response to normal levels of stress and how it responds in a healthy manner will help clarify the various ways the brain can misfire and produce debilitating symptoms and outcomes. Posttraumatic stress disorder (PTSD) is an unhealthy emotional reaction to the experienced trauma. Inability to control stress can lead to release of neurochemicals and alterations in the hypothalamic-pituitary-adrenal (HPA) axis, which can result in both central nervous system (CNS) and parasympathetic nervous system (pNs) dysregulation. The mental health provider should be familiar with various anxiety disorders, so to diagnose more accurately, because treatment differs depending upon the specific anxiety disorder diagnosis. Generalized anxiety disorder (GAD) is the most frequently diagnosed anxiety disorder in general clinics, but it is one of the least frequently diagnosed anxiety disorders in specialized anxiety clinics.