Morphologic studies of the neuropathology in Alzheimer's disease (AD) have demonstrated significant loss of synaptic connectivity in many regions of the neocortex and hippocampus. The strongest correlation with cognitive decline in AD is with the synaptic density. This article discusses the ultrastructural studies that have documented changes in synaptic numbers in many areas of association cortex and in the hippocampal dentate gyrus molecular layer. Changes in the synaptic complex are discussed as a possible compensatory mechanism in response to synapse loss and a model is proposed to help relate the significance of these synaptic changes. Comparisons are made between results observed with ultrastructural technique and those utilizing immunohistochemistry to assess changes in synaptic pathology. Possible reasons underlying the synaptic neuropathology are discussed.