Osteoarthritis is the most common joint disease worldwide. Epidemiological studies on the prevalence and incidence have documented the role of osteoarthritis in industrial populations. More extensive clinical studies not only focussed on the impact of osteoarthritis on the quality of life, but have also helped to classify this heterogeneous disease and define clinical subsets. Although the methods of epidemiological osteoarthritis research are not without problems, epidemiology has nevertheless clearly defined risk factors of osteoarthritis and evaluated environmental influences such as sports or occupation. Apart from that, epidemiology has also brought new ideas to basic science research and in particular to molecular research on the genetics of osteoarthritis.