Inlays/Onlays are made from composite resin material, porcelain, or gold.  This type of restoration is used to replaced old amalgam filings or for large restorations.

Inlay is more conservative and is used to replace the filing that only requires restoration of the chewing surface and possibly one surface between the adjacent teeth.   An Onlay is more extensive and would cover the chewing surface and both surfaces between the adjacent teeth.

Both Inlay and Onlay require two visits:

During the first visit, local anesthetic is applied and the old restoration or the decayed tooth structure is removed.  The impression of the prepared tooth is taken and sent to the dental laboratory.  Temporary material filing material is used to protect the prepared tooth.

In the second visit, the temporary filing is removed and the tooth is carefully cleaned, then the Inlay/Onlay is cemented in placed.

Once placed, it is very difficult to distinguish the Inlay/Onlay from the original tooth structure and the restoration will last a long time.