  Voids form a prominent aspect of the Megaparsec distribution of galaxies and
matter. Not only do they represent a key constituent of the Cosmic Web, they
also are one of the cleanest probes and measures of global cosmological
parameters. The shape and evolution of voids are highly sensitive to the nature
of dark energy, while their substructure and galaxy population provides a
direct key to the nature of dark matter. Also, the pristine environment of void
interiors is an important testing ground for our understanding of environmental
influences on galaxy formation and evolution. In this paper, we review the key
aspects of the structure and dynamics of voids, with a particular focus on the
hierarchical evolution of the void population. We demonstrate how the rich
structural pattern of the Cosmic Web is related to the complex evolution and
buildup of voids.
