We have reviewed research on stress effects on brain and memory processing from evolutionary, historic, and mechanistic perspectives. Our view is that the stress response has been refined through the process of natural selection to provide a rapid activation of attention and memory-related neural systems in response to a threat to survival. Specifically, stress enhances synaptic plasticity in the hippocampus (in conjunction with amygdala activation) to generate a rapid, but time-restricted, enhancement of memory. The activation period, lasting only seconds to minutes, is followed by a period in which the hippocampus is relatively resistant to developing excitatory plasticity. One consequence of this rapid, but brief, activation of the hippocampus in response to intense stress is that life-threatening experiences can produce abnormal memories which represent only small fragments of the original experience. These fragmented memories of trauma are highly resistant to extinction, and underlie the intrusive memories commonly reported in people suffering from posttraumatic stress disorder (PTSD). This evolutionary-based perspective may provide insight into the neurobiological basis of traumatic memories and aid in the development of more effective treatments for individuals diagnosed with PTSD.