The aim of this review is to point out the pathological effects after placement of subgingival crown margins or insufficiently adapted margins of restaurations. The distance between the base of the gingival sulcus and the alveolar bone crest is relatively stable in the context of periodontal health. The term "biological width" refers to the histological dimensions of epithelial and connective tissue attachment. The "biologic width" is in average 2 mm. This distance has to be respected in the case of teeth in need for restorations. From a clinical point of view a distance of at least 3 mm between the margin of restoration and the limbus alveolaris is indispensable. Several pre-prosthetic possibilities are discussed in order to maintain periodontal health. Orthodontic or surgical procedures can be used to obtain of a sufficient distance of the margin of restoration to the alveolar bone crest.