Posterior tibial tendon dysfunction (PTTD) is a complex multifaceted condition that can affect the lower extremity. Rarely mentioned 20 years ago, today it is the subject of numerous articles, books, and is a topic at most scientific seminars relating to the foot and ankle. It is a muscle imbalance initiated by a rupture, avulsion, or chronic inflammation of the tibialis posterior tendon. With time, it progresses from a flexible to rigid flatfoot deformity. Left untreated, peritalar dislocation and degenerative joint disease may develop. This article discusses the diagnosis, evaluation, and treatment of PTTD.