A cosmological theory in which the Universe has structure on all scales, from individual galaxies, to clusters of galaxies, to superclusters, and so on without end. Each level of the hierarchy is supposed to look like an enlargement of the smaller scales. A hierarchical universe is not homogeneous on large scales and hence is not consistent with the cosmological principle. Observations suggest that our Universe is approximately hierarchical on scales up to perhaps 100 million l.y., but thereafter becomes homogeneous.