This chapter examines the concept of stress and how its dysregulation leads to maladaptive outcomes and underlies several psychiatric disorders. It begins by defining different types of stress and by positioning fear and anxiety as central components of the stress response. This chapter reviews major anxiety-related disorders, including generalized anxiety disorder, panic disorder, phobic disorders, and posttraumatic stress disorder (PTSD), illustrating how stress can give rise to maladaptive behaviors. A discussion on how animal models can be used to model stress is included. The neurobiological mechanisms of dysregulated stress are discussed, detailing the function of the hypothalamic–pituitary–adrenal (HPA) axis, the involvement of brain regions like the amygdala, the frontal cortex, and the hippocampus, the role of multiple neurotransmitter systems, and the influence of the immune system on maladaptive stress responses.