Paget's disease of bone is the most common metabolic bone disease after osteoporosis and affects 2-4% of adults over 55 years of age. Its etiology is only partly understood and includes both genetic and environmental factors. The disease may be asymptomatic and can be uncovered incidentally on x-ray or in biochemical tests performed for another condition. It can also manifest itself with bone pain, deformity, fracture or other complications. Paget's disease is diagnosed by x-rays and in general has very typical radiological features, but occasionally the clinical picture may be unusual and a differential diagnosis of sclerotic or lytic metastases needs to be considered. Plasma total alkaline phosphatase activity is the most clinically useful indicator of disease activity. It is elevated in most untreated patients, but may be within the normal range in patients with monostotic or limited disease. Bisphosphonate therapy is indicated for patients with symptoms and should also be considered in patients with disease sites that suggest a risk of complications, such as long bones, vertebrae or base of the skull. Orthopedic surgery in Paget's disease patients includes almost exclusively the correction of fractures and arthroplasty.