There are many mistaken beliefs about ageing; one of them is that it is naturally accompanied by mental decline. In reality, there is a wide spectrum of changes ranging from physiological, “age-dependent” forgetfulness through slight cognitive disorders to mental decline and dementia. The most common cause of dementia is the “illness of the century”: Alzheimer’s disease, the symptoms of which people often mistakenly find in themselves, while others do not think of it even if a relative has already reached a seriously retarded mental state. There are several well-defined research models of the disease that also lay the foundations of its therapy. Dementia screening tests often identify reversible causes. Life quality can be improved by complex care even in the case of a progressive process. Mental and physical training, healthy lifestyle and control of risk factors can not only slow down ageing, but may also delay the appearance of dementia.