Adult acquired flatfoot deformity is generally associated with a collapsing medial longitudinal arch and progressive loss of strength of the tibialis posterior tendon. It is most commonly associated with posterior tibial tendon dysfunction that can have an arthritic or traumatic cause. With an increasing population of obese patients, the often misdiagnosed and overlooked posterior tibial tendon dysfunction will only continue to present more often in the foot and ankle specialist's office. This article focuses on the anatomy, classification, and pathomechanics of the flexible adult flatfoot.