Inlays and Onlays are what dentists often use if they need something “just right” to fill in gaps in teeth in molars or premolars, where a filling will not work because a tooth is too damaged, but a crown would be too intrusive. In the simplest terms, an inlay fills in cavities and hollows in a tooth in the areas between the cusps, mainly encompassing the chewing surfaces between the cusps. An onlay does more than just fill in cavities in those areas, but also covers one or more cusps of the tooth. The key comparison between them is the amount and part of the tooth that they cover. They are similar to crowns; they are both made from the same material and serve the same function, but they do not cover the entire surface of the tooth the way a crown does. If all cusps and the entire surface of the tooth are covered, this would be a crown.