The traditional way of optimizing the properties of thermosets is to tailor the density of their cross-linked network as well as the chain flexibility between the network points. Intermittent bonding is believed to improve the durability of composites. The mechanisms behind the bonding are crack deflections accompanied with long range debonding of the fibers aligned transverse to or inclined with the plane of the crack front. Information on the morphology of simultaneous interpenetrating network structures (IPNs) can be derived from differential scanning calorimetry, dynamic mechanical thermal analysis, scanning and transmission electron microscopy, and nuclear magnetic resonance proton spin diffusion studies. The morphology of IPNs is determined by mechanisms of the phase separation. The effect on cross-linking on chemorheology is markedly less studied for simultaneous VE/EP IPNs than their mechanical performance. Thermoset IPNs have been mostly used as tough, impact modified polymers, matrices in composites, and as sound and vibration damping materials.