The current state of the physics of metastable liquid systems is reviewed. The experimental data and results of theoretical calculations of the properties of liquids in the metastable region near the binodal, including the critical point, are discussed. A classification of metastable states is presented. The criteria for the possible depth of intrusion into the metastable region are discussed, and their relation to the Ginzburg criterion known in the theory of critical phenomena is studied. Especial attention is paid to the problem of the equation of state. A comparative analysis is performed of the equations of state in the metastable region, both far from the critical point and in its close vicinity. The results are discussed of studies devoted to investigating metastable systems (liquids and liquid crystals) near the spinodal. The problem is discussed of the appearance and development of fractal structures in the metastable region.