Current literature classifies popliteus tendinitis as a rare injury, yet the diagnostic criteria that distinguish this disorder from the very common running injury called iliotibial band friction syndrome are far from clear. A functional anatomical review of the popliteus muscle/tendon units (PMTU) finds several correlates that could account for the confusion. The PMTU is found to be active during most of flexion, its functions include internal rotation of the tibia on the femur, retraction of the lateral meniscus, prevention of forward displacement of the femur on the tibia, and stabilization of the knee during flexion. The etiology and clinical manifestations of several lateral knee pain disorders are very similar, and current examination techniques may not clearly distinguish the pathologic diagnosis. The suggested treatment for popliteus tendinitis is similar to other lateral knee pain disorders, further confusing any diagnosis based on the results of therapeutic trials. Further research in this area is suggested.