Dementia is more common in older age but a number of people develop symptoms at a younger age and are said to have early onset dementia (EOD). Those with EOD face different challenges to those with onset later in life. It has been difficult to quantify this disease burden. This is a systematic review of papers reporting on the prevalence of EOD. A search of Medline and Embase was performed. This was followed by a hand search of the references of these papers. Eleven suitable studies were included. All of the data was from more economically developed countries. The studies were heterogeneous in their design hindering direct comparison. The majority of the papers looked at all types of dementia although many gave a breakdown of the prevalence of different subgroups. A variety of diagnostic criteria was employed. Figures of 38 to 260 per 100 000 are quoted by papers looking at various different types of dementia together with an onset of between 30 and 64 or up to 420 per 100 000 for those aged 55–64. Prevalence rises as age approaches 65. Epidemiological data for prevalence rates for EOD are sparse. EOD remains a rare condition with low case numbers. Assimilation and comparison of results from existing studies is difficult due to methodological heterogeneity. Cross-national standardization of methodology should be a priority for future research in this area.