Adult-acquired flatfoot is flattening of the medial longitudinal arch of the foot. It was first described in 1960. In 1989, Johnson and Strom classified the condition of “posterior tibialis tendon dysfunction (PTTD)” and proposed a system of treatment. Medial longitudinal arch is stabilized by its principle stabilizer, the tendon of tibialis posterior. The plantar fascia limits spreading of the base and collapse of the apex of the arch. The posterior tibialis originates on the proximal posterior tibia, interosseous membrane and proximal fibula. The main concern of this chapter presents the anatomy and biomechanics of medial longitudinal arch, pathoanatomy of posterior tibialis tendon, pathophysiology of tibialis posterior dysfunction and resultant adult-acquired flatfoot deformity (AAFD), and clinical staging of AAFD. Clinical presentation of AAFD and management of AAFD are also discussed in this chapter.