Posterior tibial tendon dysfunction is the most common cause of acquired flatfoot deformity in adults. Although this term suggests pathology involving only the posterior tibial tendon, the disorder includes a spectrum of pathologic changes involving associated tendon, ligament, and joint structures of the ankle, hindfoot, and midfoot. Early recognition and treatment is the key to prevention of the debilitating, long-term consequences of this disorder. Conservative care is possible in the earliest stages, whereas surgical reconstruction and eventually arthrodeses become necessary in the latter stages. The purpose of this article is to review the symptoms, physical examination, radiological examination, classification, and treatment of posterior tibial tendon dysfunction.