Flat feet, also called pes planus or fallen arches, is a formal reference to a medical condition in the arch of the foot. In an estimated 20–30% of the general population, the arch simply never develops in one foot (unilaterally) or both feet (bilaterally).
Going barefoot, particularly over terrain such as a beach where muscles are given a good workout, is good for all but the most extremely flatfooted, or those with certain related conditions such as plantar fasciitis. People with Pes Plano Valgus need shoes with good arch support, such as Finn Comfort or often custom foot orthotics. This foot type should not go barefoot; wearing a supportive sandal or slipper is recommended.