Posterior view of the foot and leg, showing the Achilles tendon (tendo calcaneus). The gastrocnemius muscle is cut to expose the soleus.

Achilles' tendon or heel (tendo Achillis) or the calcaneal tendon (tendo calcaneus) is a tendon of the posterior leg. It serves to attach the gastrocnemius (calf) and soleus muscles to the calcaneus (heel) bone.

In humans it passes behind the ankle. It is the thickest and strongest tendon in the body. It is about 15 cm long, and begins near the middle of the leg, but receives fleshy fibers on its anterior surface, almost to its lower end. Gradually becoming contracted below, it is inserted into the middle part of the posterior surface of the calcaneus, a bursa being interposed between the tendon and the upper part of this surface. The tendon spreads out somewhat at its lower end, so that its narrowest part is about 4 cm. above its insertion. It is covered by the fascia and the integument, and stands out prominently behind the bone; the gap is filled up with areolar and adipose tissue. Along its lateral side, but superficial to it, is the small saphenous vein.

It gets its common name from a myth about the hero Achilles from Greek mythology. His mother Thetis decided to make him invulnerable, so she dipped him as a baby into the river Styx, whose waters had this power. However, she held the baby by his heel, and forgot to immerse that too, leaving that as his only vulnerable spot. He would later be killed during the Trojan War by a poison dart directed at his heel. This legend came to be applied to the calcaneal tendon owing to how painful it is to have it struck, and how crippling it is to have it severed. "Achilles' heel" is also a common expression for the single vulnerable spot in something or someone.