Tibialis posterior dysfunction is a complex progressive condition caused primarily by injury to the tibialis posterior tendon, leading to acquired pes planus. The tibialis posterior is the most frequent ankle tendon to be injured, and the disorder commonly occurs in late middle-aged females. Degenerative, inflammatory, functional and post-traumatic aetiologies have all been proposed. Failure of the tibialis posterior tendon causes excessive load stress on the spring ligament and sinus tarsi ligaments. A wide spectrum of bony and soft-tissue abnormalities may be seen on plain radiographs, ultrasound and MRI, including malalignment, anatomical variants, and enthesopathic and tendinopathic changes. Knowledge of the anatomical and biomechanical considerations in tibialis posterior dysfunction allows the radiologist to diagnose injury to key structures and provide prognostic information that may assist with management options to prevent further flat foot deformity.