An onlay is a dental reconstruction that covers one or more cusps of a tooth. They are usually used when a substantial reconstruction is needed. Compared to a crown more of the tooth structure can be preserved with an onlay. An onlay requires two visits; on the first visit the dentist will remove the decay or old filling, prepare the tooth, take an impression and place a temporary sealant on the tooth. The dental laboratory will then construct a custom made porcelain or gold onlay which will be fitted at the second appointment.