Diminution of cognitive ability is so frequently observed in elderly people that the terms senescence(old age) and dementia are perceived as being virtually synonymous. Although intellectual decline does not invariably accompany aging, older people are certainly at greater risk in this respect. Important research carried out within the past three decades has demonstrated that “senile dementia” is always a consequence of serious illness and not simply a function of old age. More discrete cognitive impairments such as aphasia and amnesia are also common in older populations because the elderly are more vulnerable to the medical conditions that produce them. Apart from these illness-based cognitive impairments, however, less debilitating decrements in mental functioning are also observed in older individuals and appear to reflect the aging process itself. As increasing numbers of elderly patients seek neurologic consultation for deteriorating intellectual capacities, physicians must be cognizant of the full spectrum of aging-associated cognitive disorders. To that end, the following discussion focuses on acquired mental dysfunction in later life.