Polymer nanocomposites with organoclay fillers offer improved performance and opportunities for commercial applications. The key to significant property enhancement is to exfoliate the individual organoclay platelets into the polymer matrix to utilize their high aspect ratio and modulus. The affinity between the polymer matrix and the organoclay is one of the most important factors for determining the exfoliation level. To a certain extent, the affinity can be enhanced by optimizing the organoclay structure for a given polymer matrix. Numerous studies have demonstrated that nanocomposites provide significant enhancements in stiffness and strength, flame retardancy, gas barrier properties, thermal stability and ionic conductivity. However, most polymer nanocomposites have decreased toughness relative to that of the matrix polymer. One exception to this general rule was found for nanocomposites based on poly(ethylene-co-methacrylic acid) ionomer prepared by melt compounding. My initial work investigated this system using an instrumented impact test. The data were analyzed using the essential work of fracture (EWF) methodology. Transmission electron microscopy (TEM) revealed that the clay platelets were well exfoliated in this matrix. It has also been observed that addition of organoclays to polymer blends can greatly reduce the size of the dispersed phase in some cases. It was thought that this feature might be useful for controlling rubber particle size, and, therefore, the toughness of polyamide/elastomer blends. Initially, I investigated the effect of the organoclay structure on the extent of exfoliation and properties of the nanocomposites. Nanocomposites based on the organoclays with one alkyl tail and hydroxyl ethyl groups gave well-exfoliated structures and high matrix reinforcement while nanocomposites from two-tailed organoclay contain a considerable concentration of intercalated stacks. Nanocomposites from the organoclays with one alkyl tail showed slightly better exfoliation and matrix reinforcement than those from the organoclays with hydroxyl ethyl groups. Based on this research result, the toughening response of amorphous polyamide nanocomposites using two types of elastomers, EOR and EOR-g-MA, four types of organoclays, M3(HT)1, M2(HT)2, M1H1(HT)2 and (HE)2M1T1, and two mixing protocols, has been investigated. Glass fibers (diameter ~ 12 m) are frequently used to reinforce polyamides. However, there is a practical limit to the amount of fiber that can be added while maintaining processability. Another possible use of organoclays is as an additional filler that acts on the nanoscale to complement the micro-scale reinforcement of the glass fibers. The possible synergies of simultaneous reinforcement at these very different length scales were explored and the composite moduli were compared to theoretical predictions using aspect ratios determined from TEM images. / text