Development of a multifunctional, structural material for applications in terrestrial and space-based platforms used for instrumentation in earth observation is proposed. The principal innovation is the development of an epoxy-based thermoset that undergoes a phase separation reaction during cure to form two interpenetrating networks of a structural thermoset (epoxy) phase and a second phase that is tailored to provide ancillary function. Both phases are co-continuous and nanostructured, having typical dimensions of 40-200 nm. The second phase has controllable viscoelastic properties to provide mechanical damping and other strain-rate dependent behavior. Additional functionality is obtained by sequestering appropriate materials into the second phase. Examples include reactive species for composite self-repair, ionizable salts to provide ionic conductivity, reducible and oxidizable materials for power source applications, and nanoscale materials, such as carbon nanotubes, for mechanical, electrical, and thermal properties enhancements. The thermoset will be used as a matrix in structural fiber composites. In Phase I, incorporation of functional species into the thermoset and fabrication of fiber composites will be demonstrated. In Phase II a structural composite for a specific NASA application would be developed. These multifunctional composites will reduce weight and allow incorporation of novel designs and functional features into terrestrial and space-based platforms.