The concept of employing engineering thermoplastics as toughness modifiers for Bismaleimide resins was utilized to improve the fracture toughness properties of these important materials, which have applications as matrix resins for high performance composites. Modifier molecular weight, end group functionality, backbone structure and weight percent incorporation were all studied with respect to their influence on K_lc, fracture toughness properties. Increases in fracture toughness were created with thermoplastic oligomers without sacrificing high temperature properties and desirable hot-melt processing conditions. Investigations were also made to study the morphological features that develop within these modified thermosets and their resistance to specific environments. In addition, unidirectional carbon fiber composites were prepared and their mode I and II strain energy release rates measured. Respectable increases in the interlaminar fracture toughness were obtained, 15 and 20 percent by weight loadings of maleimide terminated polysulfone modifiers yielded G_lc values of 489±25 and 734±10 J/m² respectively, a substantial improvement over the control value of 359±17 J/m². Laminates were prepared using carbon fibers that had been investigated in terms of their surface energies using Inverse Gas Chromatography. It was illustrated how this technique could distinguish between the acid-base properties of fibers possessing different degrees of proprietary surface treatments. Fiber composites containing both contrasting and subtle changes at the fiber-matrix interphase were prepared and their mechanical properties evaluated using a variety of test methods. Dramatic increases in laminate properties were measured for composites possessing contrasting interphases. Furthermore, the mode II fracture toughness test was sensitive to interphase differences; however, the mode I fracture toughness test was not. Specimens subjected to the new Continuous Ball Indentation test method (meso-indentation) were compared with single fiber micro-indentation test results. Differences were detected in composites prepared using untreated and surface treated fibers. The new method was also sensitive to changes in matrix ductility. Certain anomalies that were noted to be surprising from micro-indentation measurements were not present in the meso-indentation test results. These observations brought to light certain limitations found within the micro-indentation test, but further supported the new test method as a potential technique for fiber-matrix interphase evaluation. / Ph. D.