Onlays and inlays are restorations that are made when a filling is not an ideal option whether due to remaining tooth structure or strength. Onlays and inlays are also called indirect fillings because they are made indirectly in a dental laboratory. The process of getting one is very similar to the process of receiving a crown.
Replace a current filling
Decay/cavity that a standard filling is not ideal to replace
For an onlay or inlay to be made, an impression must be sent to a dental lab for fabrication. This is done by cleaning out the area of decay so that no decay remains. Next the area will be prepped to ideal dimensions to have a proper fitting onlay or inlay. After the area is prepped, an impression will be made of the tooth and opposing tooth so the lab will be able to correctly fabricate a proper fitting piece to be cemented in place.
After an onlay or inlay has been cemented, regular dental care will need to continue in order to prevent future decay. You should treat teeth with restorations with care and not abuse the restoration. This includes limiting the amount of very sticky foods to be chewed with the restoration, as eventually it could potentially break the cement seal of the onlay or inlay.