Dementia is a complex human disease. The incidence of dementia among the elderly population is rising rapidly worldwide. In the United States, Alzheimer's disease (AD) is the leading type of dementia and was the fifth and eighth leading cause of death in women and men aged > or = 65 years, respectively, in 2003. In Taiwan and many other counties, dementia is a hidden health issue because of its underestimation in the elderly population. In Western countries, the prevalence of AD increases from 1-3% among people aged 60-64 years to 35% among those aged > 85 years. In Taiwan, the prevalence of dementia for people aged > or = 65 years was 2-4% by 2000. Therefore, it is important to identify protective and risk factors for dementia to prevent this disease at an early stage. Several factors are related to dementia, e.g. age, ethnicity, sex, genetic factors, physical activity, smoking, drug use, education level, alcohol consumption, body mass index, comorbidity, and environmental factors. In this review, we focus on studies that have evaluated the association between these factors and the risk of dementia, especially AD and vascular dementia. We also suggest future research directions for researchers in dementia-related fields.