Osteoarthritis (OA) is the most common joint disorder to affect the knee, and the single most important cause of disability in aged elderly people. Radiological criteria for knee OA (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 that contributes to symptoms and disability in advanced disease. Nevertheless, ultrasound examination is very important in early OA of the knee when it can give information about soft tissues and cartilage involvement also in absence of clinical symptoms.