The aim of these notes is to offer a modern picture of the pertur- bative approach to the calculation of intermolecular forces. The point of view taken is that a perturbative series truncated at a low order can provide a valuable way for valuating interaction energies, especial- ly if one limits oneself to the case of intermediate- and long-range distances between the interacting partners. Although the situation corresponding to short distances is essen- tially left out from our presentation, the problems which are within the range of the theory form a vast and important class: a large var-...