No cosmological model is complete without a credible theory for the formation of the structure that is observed in the Universe. This structure, fully described in Chapter 3, ranges from very small-scale (e.g., globular clusters and dwarf galaxies), through bigger scales (e.g., galaxies and clusters of galaxies) to the largest scale (e.g., large voids, superclusters, clusters of superclusters, great walls). While a wide variety of structure formation models can and have been considered, the basic idea which has persisted since the time of Newton is gravitational instability which amplifies the growth of density fluctuations. While other scenarios are possible and will be considered here briefly, gravitationally instability has the great virtue of being 1) the only known long range force/process than can aggregate matter and 2) is physically well understood. Gravitational instability is therefore the dominant paradigm for understanding structure formation and we devote the first part of this chapter to discussing it in some detail. After this discussion we will focus on statistical methods of characterizing the distribution of structure, possible scenarios for structure formation, and finally on the available observational constraints on competing cosmogenic scenarios.