An introduction to a devastating group of diseases which cause irreversible decline in cognitive functioning. The etiological models, cognitive, emotional, and behavioral changes, treatment, care, and management issues of the most common types of dementias are explored including cortical (e.g., Alzheimer's disease, Vascular Dementia, Frontal Lobe diseases) and subcortical (e.g., Parkinson's disease, Huntington's disease, Multiple Sclerosis, and AIDS) dementias.