Several different modalities are used for clinical imaging, such as MRI, computerized tomography, positron emission tomography (PET), and single photon emission computed tomography (SPECT). In this book the emphasis is on anatomic MRI because this reflects the majority of the published literature. The major pathological features that characterize Alzheimer’s dementia include senile plaques, neurofibrillary tangles, decreased synaptic density, neuron loss, and atrophy relative to age-matched control subjects. The presumed basis of this atrophy is loss of neurons and decreased synaptic density. Measures of atrophy are divided into those assessing hemispheric atrophy and those assessing regional atrophy. Recent interest in MRI has focused on the hippocampus and other medial temporal lobe structures. MRI measurements of the hippocampus and entorhinal cortex differ significantly between control and minimally impaired groups. In other words, there is a close correlation between severity of atrophy and severity of cognitive deficits. PET and SPECT have both demonstrated abnormalities in brain perfusion, specifically in the temporoparietal region, for mildly impaired patients as well as for Alzheimer’s dementia patients.