In the UK, dementia is the leading cause of death and projections suggest there will be two million people with the syndrome by 2050. In this chapter we define dementia and describe Alzheimer’s disease, vascular dementia, dementia with Lewy bodies and frontotemporal dementia. We provide an overview of the epidemiological data which shows the prevalence of dementia both internationally and in the UK. The chapter explains the impact of dementia on countries, families, carers and individuals by discussing the financial costs of care, the challenges for carers and personal experiences. The chapter discusses the non-modifiable risk factors for dementia, which are age, genes and ethnicity. The modifiable risk factors include those which are also associated with cardiovascular disease and type 2 diabetes such as inactivity, excess weight, smoking and alcohol because of the damage they cause to blood vessels and circulation. In addition, we discuss social isolation and loneliness, intellectual inactivity and cognitive reserve, enduring depressive disorders and air pollution. We end the chapter by discussing the primary, secondary and tertiary prevention of dementia, which aims to delay cognitive decline and support a good quality of life, including screening, person-centred care and dementia-friendly communities.