The fundamental mechanisms of a metal-dielectric transition are examined. The multiband theory of the metal-dielectric transition is reviewed, as based on realistic models of the band structure of a set of oxides and sulfides of the 3d-metals in which the transition is accompanied by structural and magnetic phase transitions. In the multiband case new phases arise that were absent in the simplest single-band theories, yet at the same time are known experimentally. The metal-dielectric transition is studied in the Shubin--Vonsovskii model. It is shown that allowance for screening of the long-range Coulomb interaction leads to a first-order transition. We describe the concept of the metallic order parameter, which is proportional near the transition point to the density of states at the Fermi level. The theory enables one to explain qualitatively many experimental data, mainly thermodynamic, on the oxides of vanadium and titanium and the sulfides of 3d-metals having a metal-dielectric transition. The pathways are discussed of further development of the theory for fuller description of all physical properties of the materials being studied. Cases are analyzed in which localized magnetic moments exist and are manifested alongside the band states.