Some important unique phenomena are encountered in gels of interpenetrating polymer networks (IPN) which arise from the coupling between phase transition, elasticity, and mutual entanglements and interaction of the underlying subnetworks. The relevant order parameters of IPN-gels are obtained as the local composition ratio fluctuations, the characteristic length of microphase separation and the volume fraction of the liquid. On the basis of a preaveraged Landau-Ginzburg theory, both microphase separation and volume phase transition are considered. There exists a common critical point at which the TPN-gel becomes critical for both types of phase transitions.