The evolution of particle shape through evaporation–condensation, surface diffusion, and bulk diffusion can be described as surface motion. During the sintering of two particles, the outward surface motion near the contact increases the neck radius in the initial stage. The equations of surface velocity can be transformed into non-dimensional form by introducing a dimensionless time, leading to Herring’s scaling law. The shape evolution driven by surface tension causes the flattening of wavy surfaces, grain boundary grooving, spheroidization of particles, and Rayleigh instability of cylindrical rods and pores. The surface morphology of stressed solids evolves to decrease the total elastic strain energy through diffusion. This results in Asaro–Tiller–Grinfeld instability and the stress-induced instability of multilayers.