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Publisher Summary The brittle nature of highly crosslinked epoxy resins has limited their use in many areas. To overcome this drawback, thermally stable thermoplastics have been blended with epoxy resins in an attempt to increase their fracture toughness. One such engineering thermoplastic is polyetherimide (PEI). In this investigation, solvent-free, PEI modified epoxy-based resins capable of variable temperature cure are developed and hot-melt impregnated into woven carbon fabric. The prepreg tack, porosity in vacuum cured parts, and interlaminar fracture toughness of prepreg-based laminates are investigated as a function of PEI content in the base epoxy matrix. In this chapter, solvent free PEI epoxy blend hot-melt prepreg systems are developed that are capable of being cured with both 121°C and 177°C cure cycles while achieving similar final properties. For a prepreg system to be considered variable temperature cure, a final conversion resulting in less than 10% difference is necessary when cured at either cure cycle. Therefore, a high conversion at low temperature cure is required. It is found that relatively “dry” prepreg resin systems resulted in the lowest void content in vacuum cured parts. It is found that the PEI contained in the model resin completely phase separated when cured and formed a discontinuous phase. Through this investigation the resin, prepreg, and laminate characteristics are identified that provide the basis of variable temperature cure prepreg system necessary for high-performance applications and repair purposes.