Modern electronic and photonic devices are solid structures of small feature sizes. During fabrication and use, diusive processes can relocate matter, so that the structures evolve over time. A ยฎlm may break into droplets, and a conducting line may grow cavities. Stress and electric current have long been understood as forces that drive the changes. Evidence has accumulated that, while important, these forces are insucient to account for diverse experimental phenomena, suggesting forces of other physical origins also operate. In a structure, collective actions of atoms, electrons, and photons contribute to the free energy. When the structure changes its conยฎguration, the free energy also changes. The free energy change deยฎnes a thermodynamic force which, in its turn, drives the conยฎgurational change of the structure. This article illustrates the concepts with speciยฎc phenomena. Emphasis is placed on physical descriptions of forces of diverse origins, including elasticity, electrostatics, capillarity, electric current, composition gradient, photon dispersion, and electron conยฎnement. The eects of some of these forces are particularly signiยฎcant in structures of small feature sizes, say, between a few to hundreds of nanometers. Insights into these forces are increasingly valuable as devices miniaturize. This area of research holds great promises for solid mechanics innovation.