AbstractPatellar tendinitis, or “jumper's knee,” is a repetitive overload lesion that occurs most commonly in athletes whose sport involves eccentric loading of the patellar tendon. The prevalence of this disease is rising because of increased participation in sports, but our understanding of the underlying pathophysiology remains incomplete. The lesion shows microtears of the tendinous tissue, devitalization, and areas of focal degeneration commonly found near the bone-tendon insertion point. Though tendon imaging staging has now been proposed, staging and treatment regimens have been based solely on subjective clinical data. Therapy ranges from rest to surgical intervention. Because few controlled studies have been undertaken to examine the effectiveness of different treatments, management of the disease varies depending on the physician. Effective prevention and therapy for the future require quantitative studies of the pathophysiology of this disease.