Several incidence and prevalence studies of rheumatoid arthritis (RA) have been reported during the last decades, suggesting a considerable variation of the disease occurrence among different populations. The majority of studies carried out in Northern European and North American areas estimate a prevalence of 0.5-1%, and a mean annual incidence of 0.02-0.05%. The occurrence of the disease seems to be lower in other parts of the world. Some studies from North American, North European, and Japanese populations suggest a decline in both the prevalence and incidence of the disease after the 1960s. RA is related to an increased mortality, and the expected survival of RA patients is likely to decrease 3-10 years. There is epidemiological evidence that genetic factors are related to an increased risk of RA. However, RA is considered to be a multifactorial disease, resulting from the interaction of both genetic and environmental factors, which contribute to its occurrence and expression. The main risk factors for the disease include genetic susceptibility, sex and age, smoking, infectious agents, hormonal, dietary, socioeconomic, and ethnic factors. Most of these factors are likely to be associated with both disease occurrence and severity.