The book "In-situ Small-Angle X-ray Scattering Investigation of Transient Nanostructure of Multi-phase Polymer" by Ahmad Zeinolebadi offers a comprehensive examination of the structural properties of polymer composites. It addresses the central question in polymer materials science of why certain composites exhibit greater fatigue than pure plastics. By combining mechanical tests with small-angle X-ray scattering conducted at the DESY facility in Hamburg, the results are presented clearly and precisely. The dissertation analyzes the evolutionary changes in material structure through detailed scattering images and provides three illustrative examples of different classes of polymer composites. The findings reveal that interactions between the components can lead to unexpected structural effects, ultimately resulting in a weakening of the material where reinforcement was originally anticipated.