What is the difference between an inlay and an onlay?

Both are partial restorations made from an impression of the decayed area and then bonded in place, and the choice depends on how far the decay extends. 1) An inlay fills only the area inside the grooves of the chewing surface, when decay is limited to it. 2) An onlay is used when the decay is wider and the cusps of the chewing surface also have to be covered. If the decay is larger still, a crown covering the whole tooth may be needed. Materials include gold, ceramic, and resin, chosen with the position of the tooth and the chewing forces in mind. Which approach suits you is judged from how much tooth remains once the decay is removed.