Osteoarthritis is the commonest rheumatic disease, but despite its high prevalence in the elderly, associated disability and public health costs, there is little understanding of the aetiology of the disease. Questions still remain unanswered: 'what do we mean by the term osteoarthritis?; how do we define it?; how do we classify patients with the disease?; how do we differentiate between cases for population studies, and clinically relevant disease the physician wishes to treat?'. This chapter has demonstrated the considerable advances that have been made over the last thirty years in solving these issues, but we are still lacking a universal definition and classification of the disease. Defining and classifying OA is not an easy task, as it is not a single disease, but rather a spectrum of diseases that are the end result of a number of different processes. However, in order to understand the aetiology and natural course of the disease, and to devise appropriate treatment strategies, correct definition of OA at distinct sites is vital. Radiological site-specific definition of the disease is now recognized as the major tool available in defining OA. There is general consensus that the grading of osteophytes and joint space narrowing at the major sites using validated atlases is an important step forward.