Adhesion is one of the most frequently used words nowadays in dentistry. Although our knowledge on adhesion continues to improve daily, the situation is, however, far from understood. There is still confusion over what adhesion really is, what it means, and how it should be evaluated. The first part of this article deals with the general principles of adhesion and the specific within the dental profession over the clinical efficiency of available adhesive dentinal restorative systems. The second part of this paper presents the 2-year in-vivo results of four commercially available dentinal adhesive systems. From this trial, it seems that the more recent approach in obtaining adhesion to dentine is the most promising in terms of retentive capacities.