Interactions between closed-shell systems exhibit some common features, four of which are par-ticularly strong for beryllium bonds: geometrical distortion, cooperativity, changes in intrinsic reactivity and changes in the magnetic properties of the interacting subunits, which reflect the perturbations of their electron densities through polarization effects. Structural changes lead to interaction energies that can only be adequately accounted for when the effects of the distortion on the intrinsic reactivity of the system, and not only its deformation energy, are taken into consideration. Self-assembling of ditopic systems may lead to n-mers stabilized by strong cooperative effects. Chemical shifts and coupling constants also reflect the perturbations of the electron density and accordingly cooperative effects. These four features are common to any interaction involving two closed-shell systems, one acting as Lewis acid and the other as Lewis base, and the only difference between the nature of the interactions is quantitative.