Some individuals who experience psychologically traumatic events cannot process their experiences as regular memories, and these experiences develop into traumatic memories. In individuals with posttraumatic stress disorder (PTSD), fear conditioning occurs-these individuals subsequently respond to their direct traumatic experiences and to neutral stimuli. This conditioned response eventually leads to repeated re-experiencing. In this review, we discuss biological mechanisms associated with the development of traumatic memories. These mechanisms include neural networks connecting the amygdala, hippocampus, and ventromedial prefrontal cortex, as well as the hypothalamic-pituitary-adrenal axis of the endocrine system, and epigenetic changes.