Alzheimer's disease (AD) is an age-related disorder. Both the prevalence and the incidence of AD double approximately every 5 years after the age of 60. This is a particularly serious problem considering that the Korean elderly population is rapidly growing. We present three illustrative cases of AD in mild, moderate, and advanced stages, respectively. Their initial symptoms were memory impairment which was followed by language disturbance and visuospatial dysfunction. Abnormal behaviors such as delusion and aggression occurred in moderate and severe cases. The presence and severity of dementia in these cases were established by history taking, neurological examination, standardized mental status assessment, and neuropsychological test. Laboratory investigations showed no abnormalities that could account for the cognitive deficits. Brain CT or MRI findings of the patients wee not remarkable except for a diffuse atrophy demonstrated by increased ventricular volume, narrowed gyri, and widened sulci, As an addendum, risk factors, clinical symptoms, clinical diagnosis, and natural history of AD were reviewed.