Data on the chemistry of monocyclic, unsaturated, six-membered, oxygen-containing heterocycles, viz., 4H-pyrans, are correlated and systematized for the first time. The methods for their synthesis, their transformations, and spectroscopic data for them are discussed. The interest in these compounds, which has increased recently, is due to the fact that they are convenient starting compounds for the preparation of important classes of heterocycles such as pyrylium salts and saturated 1,5-diketones. The recyclization of the former and the cyclization of the latter lead to a great diversity of heterocycles that contain various elements (oxygen, nitrogen, sulfur, phosphorus, etc.) in five-, six-, and seven-membered rings and various substituents, including heteroaromatic and heteroorganic substituents. The preparation of 4H-pyrans is realized via three principal pathways: by condensation of carbonyl compounds, by cyclization of saturated 1,5-diketones, and by synthesis on the basis of pyrylium salts. The transformations of 4H-pyrans examined in the review include dehydroaromatization or acidic disproportionation, which lead to pyrylium salts, hydration to give saturated 1,5-diketones, reactions with the participation of the double bonds of the pyran ring, recyclization to give aromatic compounds, and the preparation of radicals and ion radicals. It was shown that 4H-pyrans are used for the solution of theoretical problems, as, for example, in the study of the mechanism of hydride transfer.