All the cosmic structures have formed from tiny fluctuations on the almost homogeneous and isotropic matter distribution in the early Universe. The initial condition of the fluctuation is supposed to be input from the cosmic inflation, which yields a single power-law power spectrum, referred to as the Harrison–Zel’dovich spectrum. The Harrison–Zel’dovich spectrum is subsequently deformed and deviates from a single power law by the cosmic expansion in the radiation-dominated epoch. This process (stagspansion) actually sets up the initial condition of the structure formation. Then, the structures of dark matter start to grow through the gravitational instability. When the density contrast is still small, the development of the structures is described by a set of linearized equations (linear regime). The high-density regions grow further and detach from the cosmic expansion (nonlinear regime), to form virialized objects (dark halos). In the present-day Universe, the large-scale structure is still in the linear regime, while clusters of galaxies and galaxies correspond to the nonlinear regime. After the decoupling between photons and baryons, the baryons fall onto the overdense regions already formed by the dark matter. This is the catch-up of baryons, which induces the onset of the formation of stars and galaxies.