The search for the possible causes of common and uncommon forms of dementia has a long and interesting history. This paper presents a brief review of how the concept of dementia has evolved over time and an historical outline of the causative factors associated with these disorders. Currently available evidence leads to the conclusion that there is only one common cause of dementia: Alzheimer's disease with or without other overlapping pathological processes (such as cerebrovascular disease, traumatic brain injury, etc.). All other forms of dementia are, in isolation, uncommon.