Adhesion is of vital interest in thin-film science because the fragile film relies on the underlying substrate, and the adhesion between the two for durability. In addition, the formation and structure of the film depend on the adhesive interaction between the depositing material and the substrate. Despite its general relevance, adhesion is one of the aspects of thin-film science about which little is known. A considerable problem arises with the measurement of adhesion, or more precisely with its lack of susceptibility to measurement. The approaches that have been used are reviewed. A further problem is concerned with the meaning and relevance of “adhesion”, and this point will be discussed from both the practical and theoretical points of view. Based as much on empiricism as on understanding, some means of modifying adhesion have been found and these also are reviewed.