One of the most common causes of acquired flatfoot deformity in adults is dysfunction of the posterior tibial tendon. The main function of the posterior tibial tendon is to invert the midfoot and lock the transverse tarsal joints (talonavicular and calcaneocuboid joints). When the tendon fails to function properly, a progressive flatfoot deformity develops. Because the disease process is a continuum, a staging system has been devised to offer guidelines for nonoperative and operative treatment of this problem. The rationale for nonoperative treatment of this disorder is to support the longitudinal arch and to decrease the valgus angulation of the calcaneus for flexible flatfoot deformity, and to immobilize and support the hindfoot and midfoot for rigid flatfoot deformities. The success of nonoperative treatment first requires the assessment of the flexibility of the flatfoot deformity. For a flexible deformity, the custom orthosis should be fitted with the foot and ankle in a corrected position as close to the neutral position as possible. Whereas, for a rigid deformity, it is imperative for the custom orthosis to be fitted with the affected foot and ankle in an in situ position.