Conventional methods of imparting toughness to ordinarily brittle thermosetting resins involve the incorporation of a second, discreet phase. Traditionally, this phase has been either a functionalized butadiene-acrylonitrile based elastomer or an unreactive thermoplastic. This dissertation describes the preparation, characterization, and evaluation of new functionalized polysiloxane elastomer and thermoplastic modifiers and their morphological implications to the toughening and physical behavior of, principally, epoxy thermosetting systems. Secondary amine-terminated poly(dimethyl-co-diphenyl siloxane) oligomers were found to be comparable tougheners to acrylonitrile-butadiene rubbers for a bisphenol-A based epoxy resin. The system that imparted the highest toughness was comprised of statistically placed 40% diphenyl and 60% dimethylsiloxane units with Mn̅ of 5000 g/mole loaded at 15% w/w. This composition resulted in a discreet second phase consisting of l μm spherical particles which were evenly dispersed throughout the cured epoxy matrix. Amine-terminated poly(arylene ether ketone) and poly(arylene ether sulfone) thermoplastics were reacted into an EPON 828/4,4'·DDS system. However, the polyketones proved to be ineffective toughening agents due to an incompatibility resulting in macroscopic phase separation. In contrast, the functionalized polysulfones were shown to be effective toughening agents, with the resultant morphology primarily a function of percent incorporation. At ~15% w/w, the polysulfone separated as l-2μm discreet particles while a 30% loading level resulted in a bicontinuous “honeycomb” morphology. The amine endgroups were shown to be necessary in controlling morphology and maximizing toughness. The polysulfone oligomers were also incorporated into a graphite fiber reinforced epoxy composite. Although improved mechanical properties were achieved, the toughness values were not as high as predicted by the neat resin evaluation. The morphology was less definable due to the complex nature and dimensions of the carbon fiber/matrix interactions. Finally, melt processing experiments indicated that amine-terminated polysulfones may act as effective processing aids for brittle bismaleimide systems, by reacting with the BMI, possibly via a Michael addition. This results in a chain extension and higher molecular weight without premature gelation occurring. / Ph. D.