Cell function is dependent of the formation and rupture of adhesive interactions. Two lines of research were followed to help understand these phenomena. Physically oriented scientists developed theoretical and experimental methods for relating the interaction energy between macroscopic surfaces to material properties of these surfaces. Biologists spent much effort in characterizing specific receptor and ligand molecules. The aim of this paper is to discuss the relationship between both approaches. It is concluded that i) so-called nonspecific and specific interactions represent combinations of the same primary forces, ii) it is however both possible and useful to discriminate specific and nonspecific interactons, and iii) as suggested by some specific examples, understanding cell behavior requires to consider both aforementioned approaches.