There is a high level of interest in using filler particles with nanometer scale for preparing composite materials with exceptional properties. Layered silicates are attractive fillers to reinforce polymers due to their relatively low cost, high strength and stiffness. The incorporation of low clay loadings (e.g., 1-5 wt%) to polymers generally leads to remarkable improvement in materials properties of the composites with respect to the neat polymers. These beneficial effects include enhanced tensile strength and stiffness, increased dimensional stability, and improved thermal property of polymers. Furthermore, lower filler loadings facilitate the composite processing and reduce the product weight. However, strain-related properties such as ductility and toughness of polymers degrade markedly as a result of the clay addition. To improve the toughness of brittle polymer-layered silicate nanocomposites, a third elastomer or rubber component must be added to the polymers. Maleated styrene-ethylene-butylene-styrene (SEBS-g-MA) is selected as a toughening agent for this purpose. This approach is similar to that adopted in toughening conventional glass fiber reinforced polymer composites. The elastomer particles act as effective craze-inducing agents during mechanical deformation, thereby promoting shear yielding of the surrounding polymer matrix. In this study, the deformation and fracture behaviors of poly(ethylene terephthalate) (PET) and thermoplastic polyolefin (TPO; 70/30 SEBS-g-MA/PP) polymers reinforced with commercially available organoclay (OMMT; Cloisite® 30B) are reported. These polymer/layered silicate nanocomposites were prepared using melt-mixing followed by injection molding. A two-step melt-mixing process was used to prepare the elastomer toughened PET/OMMT and TPO/OMMT nanocomposites. This involved an initial melt mixing of OMMT with PET or PP pellets, followed by melt compounding of PET/OMMT or PP/OMMT products with SEBS-g-MA in a twin-screw extruder. Glass fiber reinforced thermoplastics are widely used in industrial applications. However, little work has been done on the effect of layered silicate additions on the mechanical performance of glass fiber reinforced thermoplastics. In this regard, both clay and glass fiber reinforcements were incorporated into polyamide-6 (PA 6) to produce polymer hybrid composites. These hybrids were also fabricated using melt-mixing followed by injection molding. The mechanical and fracture behavior of polymer-silicate nanocomposites/ hybrids depend greatly on the microstructures of resulting polymer materials. These in-turns relate to the processing route employed, type of polymer used and dispersion state of clay platelets in polymer matrix. The structure and morphology of polymer nanocomposites and hybrids were examined by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Thermal behaviors of the nanocomposites were studied using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). XRD and TEM observations revealed the formation of intercalated structure in TPO nanocomposites and mixed intercalated/exfoliated structure in PET- or PA-based nanocomposites. The mechanical properties of these polymer nanocomposites and hybrids were determined by tensile and impact test. The effect of hygrothermal aging on the mechanical properties of such nanocomposites was also investigated. Both tensile and impact test results showed that the tensile stiffness and strength of PET/OMMT nanocomposites enhance considerably by adding low loadings of clay at the expenses of tensile ductility and impact strength. To restore tensile ductility, 10-20 wt% SEBS-g-MA elastomers were added. For the SGF/SEBS-g-MA/PA6 10/20/80 composite, OMMT addition favors stiffness enhancement but impairs its tensile ductility and impact toughness. Finally, the fracture toughness of investigated nanocomposites was determined by the essential work of fracture (EWF) approach. EWF is an effective tool to assess the fracture toughness of polymers and composites. EWF measurements revealed that the fracture toughness of TPO/OMMT nanocomposites increases with increasing clay content.