It is shown that the interplay between chemical reactions and criticality gives rise to some novel phenomena manifested both in a change of critical indices and in some pecularities in the course of chemical reactions. To cite a single example, one can mention the existence of a single point on the hypersurface of the diffusion instability where the slowing down of chemical reactions occurs. The requirements for phase separation in reactive systems are illustrated on simple models of a nonelectrolytic binary mixture and a ternary mixture including electrolytes. The general criterion for the existence of azeotropic points and the upper (lower) critical solution temperatures in reactive systems is formulated. The influence of a chemical reaction on the form of the solubility curve near the melting point in a binary and a dilute ternary mixture is analyzed in detail. A new general approach is formulated to the decay of metastable state in reactive systems. Finally, some possible experimental verifications are considered.