High-resolution electron microscopy (HREM) in conjunction with high-spatial resolution analytical electron microscopy (AEM) was used to characterize the internal interface structure and grain-boundary chemistry in SiC whisker-reinforced Si3N4-based ceramic composites. Investigations on the model system of Al2O3bicrystals are compared with studies on the complex microstructures of real composites. When relating structural features to the mechanical response of the composite, a distinction between macroscopic features, such as overall size and morphology of both matrix grains and reinforcements, and microscopic structures, such as structural and chemical width of interfaces, is made. The necessity of AEM in combination with HREM studies is shown in order to understand the performance of the composite under severe service conditions.