The arch of the foot serves as an adaptable, supportive base for the entire body. This article discusses how the arch of the foot affects the normal biomechanics of the lower limb. An anatomical overview of the three components of the arch of the foot is presented, identifying the medial longitudinal arch as the arch mainly responsible for related structural problems throughout the lower limb. Deviations in the normal structure of the medial longitudinal arch produce unbalanced, functionally unstable conditions of the foot such as pes cavus or pes planus. Specific evaluation criteria for both pes cavus and pes planus are discussed, in addition to the adverse effects these two disorders have on weight bearing, force dissipation, and normal gait. Compensatory pronation associated with pes planus is one of the most common lower extremity disorders seen currently by physical therapists working in sports medicine, and its causes and related lower limb disorders are discussed. Most of these structural deformities can be corrected through the use of various orthotic devices. Specific guidelines are presented for using both soft and permanent orthoses, which offer the foot increased shock absorption and proper structural alignment.