The present thesis explored the neurobiological basis of three aspects of defense behaviors in humans. Positron emission tomography methodology was used, and changes in regional cerebral blood flow (rCBF) were measured as an index of neural activity. Firstly, brain function was studied in a group of patients suffering from combat-related posttraumatic stress disorder, using a symptom provocation paradigm with combat sounds in order to elicit fear. Exposure to auditory trauma reminders relative to neutral sounds was associated with increased rCBF in sensorimotor areas, the cerebellar vermis, the periaqueductal gray matter, and the right amygdala, whereas decreased activity was observed in the retrosplenial area of the posterior cingulate cortex. Secondly, the neural circuitry mediating the acoustic startle response and its habituation was studied in a group of healthy subjects. During acoustic startle stimulation as compared to a resting condition, increased rCBF was found in a medial posterior area of the pons corresponding to the nucleus reticularis pontis caudalis. As a result of startle repetition, altered activity was found in the cerebellum, pointing to its involvement in startle habituation. Thirdly, neural activity associated with startle modulation by phobic fear was studied in a group of subjects with specific animal phobias during exposure to pictures of their feared and non-feared objects, paired and unpaired with acoustic startle stimuli. As a result of startle potentiation, increased rCBF was found in the left amygdaloid-hippocampal region, and medially in the affective division of the anterior cingulate cortex. In conclusion, these results provide evidence for the involvement of limbic and paralimbic brain areas during fear provocation and fear-potentiated startle and for a similar neurocircuitry underlying startle in humans and animals.