The replacement of a single missing tooth generally has two options: an implant or a bridge. 20 years ago, a bridge would be recommended. Since the improvement in technology, and the advancement in practitioner education and skill, now implants are much more widely recommended.
In the case of a bridge, the adjacent teeth must be shaved down in order to have adequate structural integrity to hold the missing tooth (pontic) in place and to make the teeth supporting the bridge durable and esthetic.
If the teeth have large restorations, caries, or an esthetic defect already, then a bridge is often the recommended treatment.
In the case where there is a tooth missing and the teeth on either side are in relatively good shape, and there is adequate bone structure to support an implant then an implant is usually recommended.