Study of the interaction and evolution behaviour of adatoms is fundamental to many surface phenomena. In the present paper, a continuum mechanics method, based on the force dipole concept, is presented to study the evolution of adatoms adsorbed on a smooth or stepped surface. The effects of elastic and van der Waals interactions are examined. It is found that the elastic interaction among adatoms usually serves as the main driving force for the clustering behaviour of adatoms and that the presence of surface defects, such as steps, may significantly influence the evolutionary arrangement of interacting adatoms.