Orthoses are externally applied devices used to modify the structural or functional characteristics of the neuro-musculo-skeletal system. They can be fitted to almost any part of the body from the leg to the arm, spine, and head, and are named systematically based on the principle joint/s they encompass. The basic functional goals of orthoses are to provide support, to correct deformity, or to modify motion occurring at a joint. Three general biomechanical principles apply to most orthotic devices: moments and force couples, pressure tolerance of tissues, and alignment of joint axes. When fabricating an orthosis, material selection depends on the function to be performed by the orthosis and with the orthosis. Metals and thermoplastics are the most common materials in modern orthotics, although thermosets are also used. Many different kinds of orthoses are available, not only with respect to the body part they encompass, but also how they are fabricated: Orthoses may be either prefabricated or custom-fabricated. Three main categories of orthoses exist: Lower limb orthoses, from the foot to the hip, generally play an important role in weight-bearing and locomotion; spinal orthoses, including cranial orthoses, are used to reduce pain, correct or decrease the progression of deformity, and prevent or minimize further injury; and upper limb orthoses, both static and dynamic, play an important role in aiding function of the upper limb, especially the hand. Within each category, many different types of orthoses are available. The main types of orthoses are described with respect to their treatment objectives, materials from which they are constructed, and mechanisms of action.