Abstract: Two types of descent mechanisms are proposed to study the stability and decay of 3-dimensional nanostructures. The first is when an upper atom can easily descend from any edge site by an any-site descent (ASD) mechanism. The other is when the atom has to descend primarily by selective-site descent (SSD) through some particular site such as a kinked site. We have studied the two different interlayer mass transport mechanisms by kinetic Monte Carlo simulation. It is found that if the islands decay primarily via ASD, an optimal island slope and a constant terrace width will be selected. In contrast, SSD gives rise to a gradual increase in the island slope. These generic conclusions provide the basis for a microscopic understanding of the morphological evolution observed in existing experimental studies of island smoothing for a variety of systems, as well as the decay of 3D Cu islands on the (100) and (111) surfaces of Cu observed in the present study.