Ultrasound of the Achilles tendon is a suitable means of differentiating various diseases of the tendon and the surrounding tissue. Different forms of degenerative disease (tendinitis, peritendinitis or bursitis, fibroosteitis, and Haglund's disease) can be discriminated from rheumatic and metabolic diseases. Congenital and developmental abnormalities can also be detected. Tendon degeneration mainly occurs in the ventral part of the medial third of the tendon ("critical zone"). Immature tissue in this area leads to temporary [correction of temorary] instability of the tendon with a high risk of rupture ("vulnerable phase"). With sonography, lesions of the Achilles tendon are visible early in the course of the disease.