In this chapter, we inspect materials that exhibit an extremely large volume- or mass-specific surface area and that can be made to expand or contract by controlling the interatomic bond strength at the surfaces and, thereby, the capillary forces. Typically, the control is achieved by manipulating space-charge regions at the surface or by inducing reversible adsorption/desorption processes. Nanoporous metals and high-surface-area carbon materials exemplify that approach.