This chapter delineates the associations between exposure to trauma, cortical grey matter loss and posttraumatic stress disorder (PTSD) symptoms. Linear relationships are shown to occur between PTSD symptoms and grey matter loss in bilateral hippocampal, anterior cingulate (including subgenual anterior cingulate cortex (ACC)), medial prefrontal cortex (mPFC) and insular cortex; in these areas, the loss is greater in those with PTSD compared with those without PTSD that have been exposed to trauma. These areas of grey matter loss are the same as the areas of decreased synaptic density that occur in the major resting state networks in PTSD: hippocampal and mPFC losses, leading to hypoconnectivity in the default mode network, and dlPFC and parietal losses, leading to a decrease in central executive network integration; however, grey matter loss in the insula, ACC and amygdala, the core nodes of the salience network, does not of itself explain the hyperactivity in this network, suggesting that a compensatory dis-inhibitory mechanism may occur in this network. Correlations are also shown to exist between combat experience, region-specific brain atrophy and specific PTSD symptom clusters. Taken together, the evidence suggests that the loss of synaptic connections, reflected in grey matter loss, disrupts functional networks in PTSD.