Diagnostic and classification guidelines for osteoarthritis based on clinical and radiographic signs for the most frequently involved joints (knee, hip and hand) have been developed by the American College of Rheumatology. Excellent, new 'high tech' biochemical and imaging methods and guidelines for clinical evaluation of the condition and its therapy have recently been developed and validated. Accordingly, we have consistent and reliable methods for assessing the established condition, but they provide little help in making an early and correct diagnosis for the different forms of osteoarthritis. Both the diagnostic process and the differential diagnosis consist of careful history taking and detailed analysis of the complaints (especially pain), with proper physical examination including a search for the source of pain and tenderness. Not all deformities and pain in and around the joints are signs and symptoms of osteoarthritis, even if the patient is of 'osteoarthritis age' and the joint shows 'osteoarthritis signs' on the x-ray. On the other hand, even if the underlying disease is osteoarthritis, the symptoms and signs may be due to disorders secondary to the basic disease (e.g. enthesopathy or tendinopathy) and the patient's complaints can be helped more easily by physiotherapy and local injections. The course of the disease and close follow-up yield diagnostic clues in cases where the cross-sectional diagnostic measures fail to provide them. Analysis of serum, joint fluid and x-ray films can be of diagnostic value, and other imaging methods (ultrasound, radioisotope scanning, computerised tomography, magnetic resonance imaging, arthroscopy) are also useful tools. Differential diagnosis is not only of theoretical value; misdiagnosis of osteoarthritis leads to either omitted or unnecessary treatment, and causes psychological stress to the patient.