Thermodynamics of glass-forming liquids and glasses is considered taking into account the contribution of heterophase fluctuations to the free energy. The role of heterophase fluctuations in liquids and glasses is played by nuclei of noncrystalline solids and nuclei of the liquid phase respectively. Crystalline solid state is not considered. Free energy is described on the bases of the droplet model and the model of mixed state. It is shown that three regions of stability of different structural states of the system exist on the(P,T)plane. Two of them are stability regions for droplet structural states, while the mixed state is stable in the third region in which heterophase fluctuations have the form of isolated drops. In this state, two infinite percolating clusters (liquid and noncrystalline solid) coexist. The width of the temperature interval[Tt1,Tt2]in which the mixed state is stable increases with configuration entropy (complexity) of the noncrystalline solid fraction. In the approach used, weak first-order phase transitions are observed atT=Tt1andT=Tt2.It is shown that first- and second-order phase transitions are possible in the mixed state. The criteria for the existence of these phase transitions are obtained. Equations of state and expressions for thermodynamic parameters (entropy, heat capacity, compressibility, and thermal expansion coefficient) are derived for the droplet and mixed states. ©1998 American Institute of Physics.