Polymeric nanocomposites have become increasingly popular in the scientific community, since the first publication 30 years ago. Due to the high surface to volume ratio of fillers with dimensions in the nanometres range, already a small amount of such fillers influences physical properties of polymers and often enhances multiple properties at once. Structural details and their relationships to the mechanical, optical and permeation properties of the nanocomposites are rather fascinating, because most physical parameters are triggered by an arrangement of the nanofiller in polymer matrix. Moreover, the low costs of clay-containing polymeric nanocomposites hold great appeal for the industrial community. The properties of a polymer nanocomposite basically depend on the behaviour of the matrix, the properties of the filler, the interactions between the filler and the polymer and finally on the arrangement of filler in matrix. Therefore, the structural details and their influence on the physical properties are very important for the system of this material. This work consists of five main chapters, aiming to successfully answer its two principal goals: characterisation of the distribution quality and structure-property relationships of clay-containing polypropylene nanocomposites. The second chapter highlights the important basics of polymeric nanocomposites, especially layered silicate reinforced polypropylene, and the basics of structural investigations by X-ray scattering techniques. The third chapter deals with the distribution quality of silicate layers in the polymer, characterized by process inline near infrared and simple X-ray scattering methods. The fourth chapter focuses on the morphological structure of the produced polymeric nanocomposites. Accordingly, the structural changes of clay-containing polypropylene nanocomposites during the production process are illustrated by the means of in situ X-ray techniques. The structure of one residual agglomerate in the nanocomposite is investigated, resulting in improved understanding of the exfoliation process during melt compounding. Chapter five focuses on the micromechanics of clay-containing polypropylene nanocomposites and the structure-properties’ relationships on a microscopic scale. Therefore, deformation mechanism and mechanics of crystallographic lattice were investigated with the help of in situ X-ray scattering tensile tests. The sixth chapter concentrates on the structure-properties’ relationships of polymeric nanocomposites on a macroscopic scale. Forasmuch, global mechanical properties (tensile test, puncture test, thermomechanical tensile test) were correlated with structural details (X-ray scattering techniques). The superstructures of layered silicate were correlated with the mobility of polymer chains and this was then related to global mechanical properties. The final chapter summarizes the major results and presents an outlook for future work.