Pes planus or flatfoot is a complex deformity seen in pediatric, adolescent, and adult populations. The medial arch of the foot collapses such that the foot is in near-complete or complete contact with the ground (Fig. 54.1). These arches are stabilized by static stabilizers (bones and ligaments) and the dynamic stabilizers (muscles and tendons). The function of the arches is to support the weight of the body, to act as shock absorption at heel strike, and to facilitate forward propulsion in the pre-swing phase of the gait cycle. The most important static and dynamic stabilizers are the talocalcaneonavicular ligament (spring ligament) and the tibialis posterior (TB).