Dementia is a decline of reasoning, memory and other mental abilities (the cognitive functions). This decline eventually impairs the ability to carry out everyday activities such as driving, household chores and even personal care such as bathing, dressing and feeding (often called activities of daily living or ADL's). Eventually, people with dementia may not recognize family members or friends and may display agitated behavior.

Dementia is most common in elderly people; it used to be called senility and was considered a normal part of aging. We now know that dementia is not a normal part of aging but is caused by a number of underlying medical conditions that can occur in both elderly and younger persons. In some cases, dementia can be reversed with proper medical treatment. In others, it is permanent and usually gets worse over time.

Dementia is a very serious condition that results in significant financial and human costs. Many people with dementia eventually become totally dependent on others for their care.

Although people with dementia typically remain fully conscious, the loss of short- and long-term memory is universal. People with dementia also experience declines in any or all areas of intellectual functioning; for example, use of language and numbers; awareness of what is going on around him or her; judgment and the ability to reason, solve problems and think abstractly. These losses not only impair a person's ability to function independently, but also have a negative impact on quality of life and relationships.

Many older people fear that they are developing dementia because they cannot find their glasses or remember someone's name. These very common problems are most often due to a much less serious condition involving slowing of mental processes with age. Medical professionals call this "benign senescent forgetfulness" or "age-related memory loss". Although this condition is a nuisance, it does not impair a person's ability to learn new information, solve problems or carry out everyday activities as dementia does.