Plasticity of synaptic connections is a critical feature of the central nervous system. Early development of the young brain and later learning and memory are all dependent upon the modulation of connections among neurons. Growing evidence suggests that disturbances of synaptic function may underlie several neuropsychiatric disorders, including epilepsy, Alzheimer's disease, and others. Two quantitative EEG (QEEG) measures, cordance and coherence, allow non-invasive assessment of synaptic dysfunction in the intact, living, human brain. We have applied these methods to the study of Alzheimer's disease and other dementing illnesses of the elderly. We summarize findings of regional brain dysfunction associated with disturbances in synaptic connectivity in these studies and suggest ways in which these techniques may be employed to assess synaptic function in investigations of normal brain development and adaptive functioning, as well as of the neurophysiology of these diseases.