Synopsis The most common pathological manifestation of fear is posttraumatic stress disorder (PTSD). Developing PTSD is closely related with predisposing factors such as genes and early traumatic experiences. In PTSD, enhanced fear learning and poor extinction are common. Fear is manifested through autonomic responses and persistent memories of the traumatic event. These manifestations are related to stress responses modulated by the hypothalamic-pituitary-adrenal axis. The current review evaluates the role of fear and stress in the course of PTSD. Findings on fear learning and extinction are presented in order to guide future treatments for patients with PTSD.