Rupture of the patellar tendon is a relatively infrequent, yet disabling, injury, which is most commonly seen in patients less than 40 years of age. It tends to occur during athletic activities when a violent contraction of the quadriceps muscle group is resisted by the flexed knee. Rupture usually represents the final stage of a degenerative tendinopathy resulting from repetitive microtrauma to the patellar tendon. This injury may also occur during less strenuous activity in patients whose tendons are weakened by systemic illness or the administration of local or systemic corticosteroid medications. The diagnosis is made on the basis of the presence of a painful, palpable defect in the substance of the tendon; an inability to completely extend the knee against gravity; and the existence of patella alta confirmed by lateral radiographs. Ultrasonography and magnetic resonance imaging are useful in identifying a neglected rupture, as well as when the diagnosis is in question or an intra-articular injury is suspected. The prognosis after a patellar tendon rupture depends in large part on the interval between injury and repair. Surgery soon after the injury is recommended for optimal results. This is best accomplished by accurate reapproximation of the ruptured tendon ends, repair of the torn extensor retinacula, and placement of a reinforcing cerclage suture. An aggressive rehabilitation program, emphasizing early range-of-motion exercises, protected weight bearing, and quadriceps strengthening, will enhance the results of surgery. Patients who undergo delayed repair are at risk for a compromised result secondary to loss of full knee flexion and decreased quadriceps strength, although a functional extensor mechanism is likely to be reestablished.