Osteoarthritis is the most common joint disorder to affect the knee and the single most important cause of disability in elderly people. Radiological criteria for osteoarthritis of the knee (focal loss of cartilage, osteophytosis, subchondral cysts and subchondral sclerosis) reflect late manifestations and significant joint damage only. Ultrasound examination is useful in revealing disruption of the periarticular tissue, which contributes to symptoms and disability in advanced disease. Nevertheless, ultrasound examination is very important in early osteoarthritis of the knee when it provides information about soft tissues and cartilage involvement in the absence of clinical symptoms.